diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0921fce..baf61b7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -34,6 +34,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - **Architecture Decision Records ADR-003 through ADR-010**: Added eight new ADRs covering FastAPI web framework (ADR-003), PostgreSQL database (ADR-004), Celery task retry strategy (ADR-005), key management in production (ADR-006), JWT authentication (ADR-007), Next.js frontend (ADR-008), Gmail API email delivery (ADR-009), and hybrid configuration model (ADR-010)
 - `userApi.updateProfile()` method in `frontend/src/lib/api.ts` for updating user profile via `PUT /users/me`
+- **Account enable/disable toggle**: `PATCH /mail-accounts/{id}/toggle` backend endpoint and a Power-icon toggle button on each account card in the UI. Disabled accounts are visually dimmed. Re-enabling an account that was in ERROR state resets its status to ACTIVE so the scheduler picks it up again.
+- **`is_enabled` checkbox in edit modal**: The Add/Edit mail account form now includes an "Enabled" checkbox so the flag can be set when creating or editing an account.
+- **Message deduplication tracking** (`DownloadedMessageId` table): Both POP3 and IMAP fetch paths now track downloaded message UIDs so the same message is never delivered twice, even when `delete_after_forward=False`.
+  - IMAP: messages are marked `\Seen` after fetching so they don't appear in future `UNSEEN` searches. DB UIDs provide a secondary guard.
+  - POP3: UIDL-based deduplication; messages are skipped if their UID is already in the DB.
+  - Old UID records are pruned by `cleanup_old_logs` after `days_to_keep` days.
+- **Gmail API "one-click" OAuth grant flow**: New `GET /providers/gmail/authorize-url` and `POST /providers/gmail/callback` endpoints. The flow requests `gmail.insert + gmail.labels` scopes with `access_type=offline` so a long-lived refresh token is issued. A new `/auth/gmail-callback` frontend page handles the redirect from Google, exchanges the code, and redirects the user back to Settings.
+- **Gmail token auto-refresh and persistence**: `GmailService` now records whether the `google-auth` library refreshed the access token during a Celery run. If it did, the Celery task writes the new access token and expiry back to `GmailCredential`, eliminating an unnecessary extra refresh call on the next run. A `401/403` or `invalid_grant` error during delivery marks `GmailCredential.is_valid = False` so the user is prompted to re-authorise.
+- **Per-user SMTP relay configuration** (`UserSmtpConfig` table): New `GET/PUT/DELETE /users/smtp-config` endpoints let each user store their own SMTP relay (host, port, username, password, TLS flag). The Celery task checks for per-user SMTP first; falls back to the global `AppSetting` SMTP config if none is set.
+- **Settings page – Gmail & SMTP sections**: The Settings page now shows a "Gmail API Delivery" card with connection status, "Connect Gmail" / "Re-authorise" / "Disconnect" buttons, and a token-lifetime explanation. An "SMTP Fallback" card lets users save their own SMTP relay credentials.
+- **Celery scheduling fix**: `process_all_enabled_accounts` previously only polled accounts with `status IN [ACTIVE, TESTING]`, causing ERROR-status accounts to be silently skipped forever. It now polls all `is_enabled = True` accounts regardless of status, so transient errors are retried automatically.
 - **Backend URL logged at startup**: The Next.js server now logs the resolved `BACKEND_URL` (e.g. `[proxy] BACKEND_URL = http://backend:8000`) via `src/instrumentation.ts` when the server starts, making it easy to diagnose `ECONNREFUSED` proxy errors. The per-request error log now also includes the full target URL.
 - **Dual-registry Docker deployment**: CI now builds separate backend and frontend images and pushes to both GHCR (`ghcr.io`) and private registry (`registry.cklnet.com`) using a matrix strategy
 - **Database-backed configuration**: `AppSetting` model and `ConfigService` for hybrid config (DB-first, env-var fallback)
diff --git a/backend/app/api/v1/endpoints/mail_accounts.py b/backend/app/api/v1/endpoints/mail_accounts.py
index a61a94c..f5f28b2 100644
--- a/backend/app/api/v1/endpoints/mail_accounts.py
+++ b/backend/app/api/v1/endpoints/mail_accounts.py
@@ -8,7 +8,7 @@ from sqlalchemy import select, desc
 from app.core.database import get_db
 from app.core.deps import get_current_active_user
 from app.core.security import encrypt_credential
-from app.models.database_models import User, MailAccount
+from app.models.database_models import User, MailAccount, AccountStatus
 from app.models.schemas import (
     MailAccountCreate,
     MailAccountResponse,
@@ -182,6 +182,38 @@ async def delete_mail_account(
     await db.commit()
 
 
+@router.patch("/{account_id}/toggle", response_model=MailAccountResponse)
+async def toggle_mail_account(
+    account_id: int,
+    current_user: User = Depends(get_current_active_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """Toggle the enabled/disabled state of a mail account"""
+    result = await db.execute(
+        select(MailAccount).where(
+            MailAccount.id == account_id, MailAccount.user_id == current_user.id
+        )
+    )
+    account = result.scalar_one_or_none()
+
+    if not account:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail="Mail account not found"
+        )
+
+    account.is_enabled = not account.is_enabled  # type: ignore[assignment]
+
+    # When re-enabling a previously errored account, reset status to ACTIVE
+    # so the scheduler picks it up on the next run.
+    if account.is_enabled and account.status == AccountStatus.ERROR:
+        account.status = AccountStatus.ACTIVE  # type: ignore[assignment]
+
+    await db.commit()
+    await db.refresh(account)
+
+    return account
+
+
 @router.post("/test", response_model=MailAccountTestResponse)
 async def test_mail_connection(
     test_request: MailAccountTestRequest,
diff --git a/backend/app/api/v1/endpoints/providers.py b/backend/app/api/v1/endpoints/providers.py
index d6603fd..6ef5ea5 100644
--- a/backend/app/api/v1/endpoints/providers.py
+++ b/backend/app/api/v1/endpoints/providers.py
@@ -1,10 +1,12 @@
 """Provider presets and Gmail credential management endpoints"""
 
-from datetime import datetime, timezone
-from typing import List
+from datetime import datetime, timedelta, timezone
+from typing import List, Optional
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
+import httpx
+import logging
 
 from app.core.database import get_db
 from app.core.deps import get_current_active_user
@@ -16,10 +18,21 @@ from app.models.schemas import (
     ProviderListResponse,
     GmailCredentialCreate,
     GmailCredentialResponse,
+    GmailAuthorizeResponse,
+    GmailCallbackRequest,
 )
 from app.services.gmail_service import GmailService
 
 router = APIRouter()
+logger = logging.getLogger(__name__)
+
+# Gmail API scopes needed for email injection
+GMAIL_API_SCOPES = [
+    "openid",
+    "email",
+    "https://www.googleapis.com/auth/gmail.insert",
+    "https://www.googleapis.com/auth/gmail.labels",
+]
 
 # Provider presets with server configurations
 PROVIDER_PRESETS: List[ProviderPreset] = [
@@ -268,3 +281,166 @@ async def delete_gmail_credential(
 
     await db.delete(credential)
     await db.commit()
+
+
+@router.get("/gmail/authorize-url", response_model=GmailAuthorizeResponse)
+async def get_gmail_authorize_url(
+    redirect_uri: str,
+    current_user: User = Depends(get_current_active_user),
+):
+    """
+    Return a Google OAuth2 URL that grants this app Gmail API write permissions.
+
+    The URL requests the gmail.insert + gmail.labels scopes with
+    access_type=offline so a long-lived refresh token is issued.
+    prompt=consent forces Google to always issue a new refresh token even if
+    the user has authorised the app before.
+    """
+    if not settings.GOOGLE_CLIENT_ID:
+        raise HTTPException(
+            status_code=status.HTTP_501_NOT_IMPLEMENTED,
+            detail="Google OAuth2 is not configured on this server.",
+        )
+
+    scope = " ".join(GMAIL_API_SCOPES)
+    url = (
+        "https://accounts.google.com/o/oauth2/v2/auth"
+        f"?client_id={settings.GOOGLE_CLIENT_ID}"
+        "&response_type=code"
+        f"&scope={scope}"
+        f"&redirect_uri={redirect_uri}"
+        "&access_type=offline"
+        "&prompt=consent"
+    )
+    return GmailAuthorizeResponse(authorization_url=url)
+
+
+@router.post(
+    "/gmail/callback",
+    response_model=GmailCredentialResponse,
+    status_code=status.HTTP_201_CREATED,
+)
+async def gmail_oauth_callback(
+    callback_in: GmailCallbackRequest,
+    current_user: User = Depends(get_current_active_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """
+    Exchange a Google authorization code for Gmail API tokens and persist them.
+
+    This is the backend half of the "Connect Gmail" one-click flow.  The
+    frontend redirects the user to Google, Google sends a code back to the
+    frontend callback page, and the frontend posts that code here.
+
+    Token lifetime
+    --------------
+    * Access token  : 1 hour.  The google-auth library auto-refreshes it
+      during Celery tasks; the new value is written back to this row so the
+      next run doesn't need an extra refresh round-trip.
+    * Refresh token : does not expire unless the user revokes access or the
+      app credentials change.  If revoked, ``is_valid`` is set to False by
+      the next Celery run that hits a 401, and the user must re-authorise.
+    """
+    if not settings.GOOGLE_CLIENT_ID or not settings.GOOGLE_CLIENT_SECRET:
+        raise HTTPException(
+            status_code=status.HTTP_501_NOT_IMPLEMENTED,
+            detail="Google OAuth2 is not configured on this server.",
+        )
+
+    # Exchange code for tokens
+    async with httpx.AsyncClient() as client:
+        token_resp = await client.post(
+            "https://oauth2.googleapis.com/token",
+            data={
+                "code": callback_in.code,
+                "client_id": settings.GOOGLE_CLIENT_ID,
+                "client_secret": settings.GOOGLE_CLIENT_SECRET,
+                "redirect_uri": callback_in.redirect_uri,
+                "grant_type": "authorization_code",
+            },
+        )
+
+    if token_resp.status_code != 200:
+        logger.error(f"Gmail token exchange failed: {token_resp.text}")
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Failed to exchange authorization code with Google.",
+        )
+
+    token_data = token_resp.json()
+    access_token: Optional[str] = token_data.get("access_token")
+    refresh_token: Optional[str] = token_data.get("refresh_token")
+
+    if not access_token:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Google did not return an access token.",
+        )
+
+    # Fetch the Gmail email address to associate with this credential
+    async with httpx.AsyncClient() as client:
+        profile_resp = await client.get(
+            "https://www.googleapis.com/oauth2/v2/userinfo",
+            headers={"Authorization": f"Bearer {access_token}"},
+        )
+
+    gmail_email = current_user.email  # fallback
+    if profile_resp.status_code == 200:
+        gmail_email = profile_resp.json().get("email", current_user.email)
+
+    # Calculate token expiry (Google access tokens last 1 hour)
+    token_expiry = datetime.now(timezone.utc) + timedelta(
+        seconds=int(token_data.get("expires_in", 3600))
+    )
+
+    # Verify the credentials actually work with the Gmail API
+    gmail_service = GmailService(
+        access_token=access_token,
+        refresh_token=refresh_token,
+        client_id=settings.GOOGLE_CLIENT_ID,
+        client_secret=settings.GOOGLE_CLIENT_SECRET,
+    )
+    is_valid = await gmail_service.verify_access()
+    if not is_valid:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Obtained tokens but could not verify Gmail API access. "
+            "Ensure the gmail.insert scope was granted.",
+        )
+
+    # Persist tokens
+    result = await db.execute(
+        select(GmailCredential).where(GmailCredential.user_id == current_user.id)
+    )
+    existing = result.scalar_one_or_none()
+
+    encrypted_access = encrypt_credential(access_token)
+    encrypted_refresh = encrypt_credential(refresh_token) if refresh_token else None
+
+    if existing:
+        existing.gmail_email = gmail_email  # type: ignore[assignment]
+        existing.encrypted_access_token = encrypted_access  # type: ignore[assignment]
+        if encrypted_refresh:
+            existing.encrypted_refresh_token = encrypted_refresh  # type: ignore[assignment]
+        existing.token_expiry = token_expiry  # type: ignore[assignment]
+        existing.scopes = token_data.get("scope", "").split()  # type: ignore[assignment]
+        existing.is_valid = True  # type: ignore[assignment]
+        existing.last_verified_at = datetime.now(timezone.utc)  # type: ignore[assignment]
+        await db.commit()
+        await db.refresh(existing)
+        return existing
+    else:
+        credential = GmailCredential(
+            user_id=current_user.id,
+            gmail_email=gmail_email,
+            encrypted_access_token=encrypted_access,
+            encrypted_refresh_token=encrypted_refresh,
+            token_expiry=token_expiry,
+            scopes=token_data.get("scope", "").split(),
+            is_valid=True,
+            last_verified_at=datetime.now(timezone.utc),
+        )
+        db.add(credential)
+        await db.commit()
+        await db.refresh(credential)
+        return credential
diff --git a/backend/app/api/v1/endpoints/users.py b/backend/app/api/v1/endpoints/users.py
index a609652..60c05be 100644
--- a/backend/app/api/v1/endpoints/users.py
+++ b/backend/app/api/v1/endpoints/users.py
@@ -1,12 +1,19 @@
 """User management endpoints"""
 
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select
 
 from app.core.database import get_db
 from app.core.deps import get_current_active_user
-from app.models.database_models import User
-from app.models.schemas import UserDetailResponse, UserUpdate
+from app.core.security import encrypt_credential
+from app.models.database_models import User, UserSmtpConfig
+from app.models.schemas import (
+    UserDetailResponse,
+    UserUpdate,
+    UserSmtpConfigUpdate,
+    UserSmtpConfigResponse,
+)
 
 router = APIRouter()
 
@@ -34,3 +41,96 @@ async def update_current_user_profile(
     await db.commit()
     await db.refresh(current_user)
     return current_user
+
+
+@router.get("/smtp-config", response_model=UserSmtpConfigResponse)
+async def get_smtp_config(
+    current_user: User = Depends(get_current_active_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """Get the current user's SMTP relay configuration"""
+    result = await db.execute(
+        select(UserSmtpConfig).where(UserSmtpConfig.user_id == current_user.id)
+    )
+    config = result.scalar_one_or_none()
+
+    if not config:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="No SMTP configuration found. Save one first.",
+        )
+
+    return UserSmtpConfigResponse(
+        id=config.id,  # type: ignore[arg-type]
+        user_id=config.user_id,  # type: ignore[arg-type]
+        host=config.host,  # type: ignore[arg-type]
+        port=config.port,  # type: ignore[arg-type]
+        username=config.username,  # type: ignore[arg-type]
+        use_tls=config.use_tls,  # type: ignore[arg-type]
+        has_password=bool(config.encrypted_password),
+        created_at=config.created_at,  # type: ignore[arg-type]
+        updated_at=config.updated_at,  # type: ignore[arg-type]
+    )
+
+
+@router.put("/smtp-config", response_model=UserSmtpConfigResponse)
+async def upsert_smtp_config(
+    config_in: UserSmtpConfigUpdate,
+    current_user: User = Depends(get_current_active_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """Create or update the current user's SMTP relay configuration"""
+    result = await db.execute(
+        select(UserSmtpConfig).where(UserSmtpConfig.user_id == current_user.id)
+    )
+    config = result.scalar_one_or_none()
+
+    if config:
+        config.host = config_in.host  # type: ignore[assignment]
+        config.port = config_in.port  # type: ignore[assignment]
+        config.username = config_in.username  # type: ignore[assignment]
+        config.use_tls = config_in.use_tls  # type: ignore[assignment]
+        if config_in.password is not None:
+            config.encrypted_password = encrypt_credential(config_in.password)  # type: ignore[assignment]
+    else:
+        config = UserSmtpConfig(
+            user_id=current_user.id,
+            host=config_in.host,
+            port=config_in.port,
+            username=config_in.username,
+            encrypted_password=(
+                encrypt_credential(config_in.password) if config_in.password else ""
+            ),
+            use_tls=config_in.use_tls,
+        )
+        db.add(config)
+
+    await db.commit()
+    await db.refresh(config)
+
+    return UserSmtpConfigResponse(
+        id=config.id,  # type: ignore[arg-type]
+        user_id=config.user_id,  # type: ignore[arg-type]
+        host=config.host,  # type: ignore[arg-type]
+        port=config.port,  # type: ignore[arg-type]
+        username=config.username,  # type: ignore[arg-type]
+        use_tls=config.use_tls,  # type: ignore[arg-type]
+        has_password=bool(config.encrypted_password),
+        created_at=config.created_at,  # type: ignore[arg-type]
+        updated_at=config.updated_at,  # type: ignore[arg-type]
+    )
+
+
+@router.delete("/smtp-config", status_code=status.HTTP_204_NO_CONTENT)
+async def delete_smtp_config(
+    current_user: User = Depends(get_current_active_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """Delete the current user's SMTP relay configuration"""
+    result = await db.execute(
+        select(UserSmtpConfig).where(UserSmtpConfig.user_id == current_user.id)
+    )
+    config = result.scalar_one_or_none()
+    if config:
+        await db.delete(config)
+        await db.commit()
diff --git a/backend/app/models/database_models.py b/backend/app/models/database_models.py
index 3bf2126..7932792 100644
--- a/backend/app/models/database_models.py
+++ b/backend/app/models/database_models.py
@@ -446,6 +446,76 @@ class AuditLog(Base):
     )
 
 
+class UserSmtpConfig(Base):
+    """Per-user SMTP relay configuration for email forwarding fallback."""
+
+    __tablename__ = "user_smtp_configs"
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, unique=True
+    )
+
+    host = Column(String(255), nullable=False, default="smtp.gmail.com")
+    port = Column(Integer, nullable=False, default=587)
+    username = Column(String(255), nullable=False, default="")
+    encrypted_password = Column(Text, nullable=False, default="")
+    use_tls = Column(Boolean, default=True)
+
+    created_at = Column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+    updated_at = Column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+
+    # Relationships
+    user = relationship("User", backref="smtp_config")
+
+
+class DownloadedMessageId(Base):
+    """
+    Tracks unique message IDs that have already been downloaded and forwarded.
+
+    - For POP3: stores the UIDL string returned by the server.
+    - For IMAP: stores the IMAP UID (numeric string) of the message.
+
+    This prevents re-processing the same message when delete_after_forward=False.
+    """
+
+    __tablename__ = "downloaded_message_ids"
+
+    id = Column(Integer, primary_key=True, index=True)
+    mail_account_id = Column(
+        Integer, ForeignKey("mail_accounts.id", ondelete="CASCADE"), nullable=False
+    )
+
+    # Unique message identifier (UIDL for POP3, UID for IMAP)
+    message_uid = Column(String(512), nullable=False)
+
+    downloaded_at = Column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+
+    # Indexes — unique constraint prevents duplicates
+    __table_args__ = (
+        Index(
+            "idx_account_message_uid",
+            "mail_account_id",
+            "message_uid",
+            unique=True,
+        ),
+        Index("idx_downloaded_at", "downloaded_at"),
+    )
+
+
 class GmailCredential(Base):
     """Stores OAuth2 credentials for Gmail API access (per-user)"""
 
diff --git a/backend/app/models/schemas.py b/backend/app/models/schemas.py
index 8261291..6321eb0 100644
--- a/backend/app/models/schemas.py
+++ b/backend/app/models/schemas.py
@@ -339,3 +339,35 @@ class ProviderPreset(BaseModel):
 
 class ProviderListResponse(BaseModel):
     providers: List[ProviderPreset]
+
+
+# User SMTP Config Schemas
+class UserSmtpConfigBase(BaseModel):
+    host: str = "smtp.gmail.com"
+    port: int = Field(587, gt=0, lt=65536)
+    username: str = ""
+    use_tls: bool = True
+
+
+class UserSmtpConfigUpdate(UserSmtpConfigBase):
+    password: Optional[str] = None  # Only provided when changing the password
+
+
+class UserSmtpConfigResponse(UserSmtpConfigBase):
+    id: int
+    user_id: int
+    has_password: bool  # True if a password is stored (value is never returned)
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+# Gmail OAuth Schemas
+class GmailAuthorizeResponse(BaseModel):
+    authorization_url: str
+
+
+class GmailCallbackRequest(BaseModel):
+    code: str
+    redirect_uri: str
diff --git a/backend/app/services/gmail_service.py b/backend/app/services/gmail_service.py
index 403bf46..717fba1 100644
--- a/backend/app/services/gmail_service.py
+++ b/backend/app/services/gmail_service.py
@@ -56,6 +56,7 @@ class GmailService:
             client_id: Google OAuth2 client ID
             client_secret: Google OAuth2 client secret
         """
+        self._initial_access_token = access_token
         self.credentials = Credentials(
             token=access_token,
             refresh_token=refresh_token,
@@ -135,6 +136,7 @@ class GmailService:
                 f"Gmail API error: {e.reason if hasattr(e, 'reason') else str(e)}"
             )
             logger.error(error_msg)
+            # Surface 401 so callers can mark credentials as invalid
             raise GmailInjectionError(error_msg)
         except Exception as e:
             error_msg = f"Failed to inject email into Gmail: {str(e)}"
@@ -180,3 +182,24 @@ class GmailService:
         except Exception as e:
             logger.error(f"Failed to get Gmail email address: {e}")
             return None
+
+    def get_refreshed_token(self) -> Optional[Dict[str, Any]]:
+        """
+        Return the current access token and expiry if the token was refreshed
+        since this service instance was created.
+
+        The google-auth library auto-refreshes the access token when an API
+        call is made with an expired token.  Call this after inject_email() to
+        check whether a refresh happened and persist the new token.
+
+        Returns:
+            Dict with ``access_token`` and ``expiry`` (datetime | None), or
+            None if the token has not changed from the one passed to __init__.
+        """
+        current_token = self.credentials.token
+        if current_token and current_token != self._initial_access_token:
+            return {
+                "access_token": current_token,
+                "expiry": self.credentials.expiry,
+            }
+        return None
diff --git a/backend/app/services/mail_processor.py b/backend/app/services/mail_processor.py
index 9829bf8..144e6a5 100644
--- a/backend/app/services/mail_processor.py
+++ b/backend/app/services/mail_processor.py
@@ -11,7 +11,7 @@ from email import parser
 from email.mime.text import MIMEText
 from email.mime.multipart import MIMEMultipart
 from email.utils import formatdate, make_msgid
-from typing import List, Dict, Any, Optional, Tuple
+from typing import List, Dict, Any, Optional, Set, Tuple
 import logging
 from aioimaplib import aioimaplib
 
@@ -143,26 +143,42 @@ class MailProcessor:
         except Exception as e:
             return False, f"IMAP connection failed: {str(e)}"
 
-    async def fetch_emails(self, max_count: Optional[int] = None) -> List[bytes]:
+    async def fetch_emails(
+        self,
+        max_count: Optional[int] = None,
+        already_seen_uids: Optional[Set[str]] = None,
+    ) -> Tuple[List[bytes], List[str]]:
         """
         Fetch emails from the mail server.
-        Returns list of raw email data.
+
+        Args:
+            max_count: Maximum number of messages to fetch.
+            already_seen_uids: Set of message UIDs that have already been
+                processed and should be skipped.
+
+        Returns:
+            A tuple of (raw_email_bytes_list, new_uid_strings_list).
+            The caller should persist the new UIDs to prevent re-processing.
         """
         effective_max: int = max_count if max_count is not None else self.account.max_emails_per_check  # type: ignore[assignment]
+        seen: Set[str] = already_seen_uids or set()
 
         if self.account.protocol in [MailProtocol.POP3, MailProtocol.POP3_SSL]:
-            return await self._fetch_pop3_emails(effective_max)
+            return await self._fetch_pop3_emails(effective_max, seen)
         else:
-            return await self._fetch_imap_emails(effective_max)
+            return await self._fetch_imap_emails(effective_max, seen)
 
-    async def _fetch_pop3_emails(self, max_count: int) -> List[bytes]:
-        """Fetch emails via POP3"""
-        emails = []
+    async def _fetch_pop3_emails(
+        self, max_count: int, already_seen_uids: Set[str]
+    ) -> Tuple[List[bytes], List[str]]:
+        """Fetch emails via POP3, skipping already-downloaded UIDs."""
+        emails: List[bytes] = []
+        new_uids: List[str] = []
 
         try:
             loop = asyncio.get_event_loop()
 
-            def fetch_pop3():
+            def fetch_pop3() -> Tuple[List[bytes], List[str]]:
                 # Connect
                 if self.account.protocol == MailProtocol.POP3_SSL:
                     context = ssl.create_default_context()
@@ -181,27 +197,49 @@ class MailProcessor:
                 pop_conn.user(self.account.username)
                 pop_conn.pass_(self.password)
 
-                # Get message count
-                num_messages = len(pop_conn.list()[1])
+                # Retrieve UIDL map: {msg_number: uid_string}
+                uidl_response = pop_conn.uidl()
+                uid_map: Dict[int, str] = {}
+                for entry in uidl_response[1]:
+                    parts = entry.decode().split(" ", 1)
+                    if len(parts) == 2:
+                        uid_map[int(parts[0])] = parts[1].strip()
+
+                num_messages = len(uid_map)
                 logger.info(
                     f"Found {num_messages} messages for account {self.account.id}"
                 )
 
-                fetched_emails = []
-                messages_to_delete = []
+                fetched: List[bytes] = []
+                fetched_uids: List[str] = []
+                messages_to_delete: List[int] = []
+                fetched_count = 0
+
+                for msg_num, uid in uid_map.items():
+                    if fetched_count >= max_count:
+                        break
+
+                    # Skip messages we already processed
+                    if uid in already_seen_uids:
+                        logger.debug(
+                            f"Skipping already-downloaded message {uid} "
+                            f"for account {self.account.id}"
+                        )
+                        continue
 
-                # Fetch emails (limited by max_count)
-                for i in range(1, min(num_messages + 1, max_count + 1)):
                     try:
-                        response, lines, octets = pop_conn.retr(i)
+                        response, lines, octets = pop_conn.retr(msg_num)
                         email_data = b"\r\n".join(lines)
-                        fetched_emails.append(email_data)
-                        messages_to_delete.append(i)
+                        fetched.append(email_data)
+                        fetched_uids.append(uid)
+                        messages_to_delete.append(msg_num)
+                        fetched_count += 1
                         logger.info(
-                            f"Retrieved message {i} from account {self.account.id}"
+                            f"Retrieved message {msg_num} (uid={uid}) "
+                            f"from account {self.account.id}"
                         )
                     except Exception as e:
-                        logger.error(f"Error retrieving message {i}: {e}")
+                        logger.error(f"Error retrieving message {msg_num}: {e}")
 
                 # Delete messages if configured
                 if self.account.delete_after_forward:
@@ -212,19 +250,22 @@ class MailProcessor:
                             logger.error(f"Error deleting message {msg_id}: {e}")
 
                 pop_conn.quit()
-                return fetched_emails
+                return fetched, fetched_uids
 
-            emails = await loop.run_in_executor(None, fetch_pop3)
+            emails, new_uids = await loop.run_in_executor(None, fetch_pop3)
 
         except Exception as e:
             logger.error(f"Error fetching POP3 emails: {e}")
             raise MailFetchError(f"POP3 fetch error: {str(e)}")
 
-        return emails
+        return emails, new_uids
 
-    async def _fetch_imap_emails(self, max_count: int) -> List[bytes]:
-        """Fetch emails via IMAP"""
-        emails = []
+    async def _fetch_imap_emails(
+        self, max_count: int, already_seen_uids: Set[str]
+    ) -> Tuple[List[bytes], List[str]]:
+        """Fetch emails via IMAP, marking each message \Seen to prevent re-fetch."""
+        emails: List[bytes] = []
+        new_uids: List[str] = []
 
         try:
             # Create IMAP client
@@ -241,8 +282,8 @@ class MailProcessor:
             await imap_client.login(self.account.username, self.password)
             await imap_client.select("INBOX")
 
-            # Search for all messages
-            response = await imap_client.search("UNSEEN")  # Only fetch unread
+            # Search for unseen messages only
+            response = await imap_client.search("UNSEEN")
             message_ids = response.lines[0].split()
 
             # Limit to max_count
@@ -254,6 +295,18 @@ class MailProcessor:
 
             # Fetch each message
             for msg_id in message_ids:
+                uid_str = msg_id.decode() if isinstance(msg_id, bytes) else str(msg_id)
+
+                # Skip messages already tracked in our DB
+                if uid_str in already_seen_uids:
+                    logger.debug(
+                        f"Skipping already-processed IMAP message {uid_str} "
+                        f"for account {self.account.id}"
+                    )
+                    # Still mark as Seen so it doesn't show up in UNSEEN searches
+                    await imap_client.store(msg_id, "+FLAGS", "\\Seen")
+                    continue
+
                 try:
                     response = await imap_client.fetch(msg_id, "(RFC822)")
 
@@ -272,8 +325,12 @@ class MailProcessor:
 
                     if email_data:
                         emails.append(email_data)
+                        new_uids.append(uid_str)
+
+                        # Always mark as Seen after fetching so the message is
+                        # not picked up again on the next UNSEEN search.
+                        await imap_client.store(msg_id, "+FLAGS", "\\Seen")
 
-                        # Mark as seen if deleting after forward
                         if self.account.delete_after_forward:
                             await imap_client.store(msg_id, "+FLAGS", "\\Deleted")
 
@@ -290,7 +347,7 @@ class MailProcessor:
             logger.error(f"Error fetching IMAP emails: {e}")
             raise MailFetchError(f"IMAP fetch error: {str(e)}")
 
-        return emails
+        return emails, new_uids
 
     @staticmethod
     async def forward_email(
diff --git a/backend/app/workers/tasks.py b/backend/app/workers/tasks.py
index 4038659..da4891a 100644
--- a/backend/app/workers/tasks.py
+++ b/backend/app/workers/tasks.py
@@ -9,7 +9,7 @@ import logging
 
 from app.workers.celery_app import celery_app
 from app.core.database import async_session_maker
-from app.core.security import decrypt_credential
+from app.core.security import decrypt_credential, encrypt_credential
 from app.models.database_models import (
     MailAccount,
     ProcessingRun,
@@ -17,12 +17,14 @@ from app.models.database_models import (
     AccountStatus,
     DeliveryMethod,
     GmailCredential,
+    DownloadedMessageId,
+    UserSmtpConfig,
 )
 from app.services.mail_processor import MailProcessor
 from app.services.gmail_service import GmailService
 from app.services.config_service import ConfigService
 from app.core.config import settings
-from sqlalchemy import select, and_
+from sqlalchemy import select, delete
 
 logger = logging.getLogger(__name__)
 
@@ -69,11 +71,22 @@ async def process_mail_account(account_id: int):
             # Decrypt password
             password = decrypt_credential(account.encrypted_password)  # type: ignore[arg-type]
 
+            # Load already-downloaded UIDs to prevent re-processing
+            seen_result = await db.execute(
+                select(DownloadedMessageId.message_uid).where(
+                    DownloadedMessageId.mail_account_id == account.id
+                )
+            )
+            already_seen_uids = set(seen_result.scalars().all())
+
             # Create processor
             processor = MailProcessor(account, password)
 
-            # Fetch emails
-            emails = await processor.fetch_emails(account.max_emails_per_check)  # type: ignore[arg-type]
+            # Fetch emails (returns raw bytes + new UIDs)
+            emails, new_uids = await processor.fetch_emails(
+                account.max_emails_per_check,  # type: ignore[arg-type]
+                already_seen_uids=already_seen_uids,
+            )
 
             run.emails_fetched = len(emails)  # type: ignore[assignment]
 
@@ -118,8 +131,24 @@ async def process_mail_account(account_id: int):
                     use_gmail_api = False  # type: ignore[assignment]
 
             if not use_gmail_api:
-                # Fall back to SMTP – read config from DB with env fallback
-                smtp_config = await ConfigService.get_smtp_config(db=db)
+                # Fall back to SMTP – check per-user config first, then global
+                user_smtp_result = await db.execute(
+                    select(UserSmtpConfig).where(
+                        UserSmtpConfig.user_id == account.user_id
+                    )
+                )
+                user_smtp = user_smtp_result.scalar_one_or_none()
+
+                if user_smtp and user_smtp.username and user_smtp.encrypted_password:
+                    smtp_config = {
+                        "host": user_smtp.host,
+                        "port": user_smtp.port,
+                        "username": user_smtp.username,
+                        "password": decrypt_credential(user_smtp.encrypted_password),  # type: ignore[arg-type]
+                        "use_tls": user_smtp.use_tls,
+                    }
+                else:
+                    smtp_config = await ConfigService.get_smtp_config(db=db)
 
                 if not smtp_config["username"] or not smtp_config["password"]:
                     logger.error(
@@ -130,7 +159,15 @@ async def process_mail_account(account_id: int):
                     await db.commit()
                     return
 
-            for email_data in emails:
+            successfully_forwarded_uids: list[str] = []
+
+            if len(emails) != len(new_uids):
+                logger.error(
+                    f"emails/uids length mismatch ({len(emails)} vs {len(new_uids)}) "
+                    f"for account {account.id}; truncating to shorter list"
+                )
+
+            for email_data, uid in zip(emails, new_uids):
                 try:
                     if use_gmail_api and gmail_service:
                         # Inject via Gmail API (preferred)
@@ -140,6 +177,7 @@ async def process_mail_account(account_id: int):
                             source_account_name=account.name,  # type: ignore[arg-type]
                         )
                         emails_forwarded += 1
+                        successfully_forwarded_uids.append(uid)
                     else:
                         # Forward via SMTP (fallback)
                         success = await MailProcessor.forward_email(
@@ -147,13 +185,54 @@ async def process_mail_account(account_id: int):
                         )
                         if success:
                             emails_forwarded += 1
+                            successfully_forwarded_uids.append(uid)
                         else:
                             emails_failed += 1
 
                 except Exception as e:
+                    error_str = str(e).lower()
+                    # If Gmail returns 401/403 the refresh token was revoked –
+                    # mark credentials invalid so the user gets notified.
+                    if (
+                        use_gmail_api
+                        and gmail_cred
+                        and (
+                            "401" in error_str
+                            or "403" in error_str
+                            or "invalid_grant" in error_str
+                        )
+                    ):
+                        gmail_cred.is_valid = False  # type: ignore[assignment]
+                        logger.warning(
+                            f"Gmail credentials revoked for user {account.user_id}. "
+                            "User must re-authorise."
+                        )
                     logger.error(f"Error delivering email: {e}")
                     emails_failed += 1
 
+            # Persist new message UIDs so they are not processed again
+            for uid in successfully_forwarded_uids:
+                if uid not in already_seen_uids:
+                    db.add(
+                        DownloadedMessageId(
+                            mail_account_id=account.id,
+                            message_uid=uid,
+                        )
+                    )
+
+            # If Gmail API was used, persist any refreshed access token back to
+            # the DB so the next run doesn't need an extra token-refresh call.
+            if use_gmail_api and gmail_service and gmail_cred:
+                refreshed = gmail_service.get_refreshed_token()
+                if refreshed and refreshed["access_token"] != access_token:
+                    gmail_cred.encrypted_access_token = encrypt_credential(refreshed["access_token"])  # type: ignore[assignment]
+                    if refreshed.get("expiry"):
+                        gmail_cred.token_expiry = refreshed["expiry"]  # type: ignore[assignment]
+                    gmail_cred.last_verified_at = datetime.now(timezone.utc)  # type: ignore[assignment]
+                    logger.info(
+                        f"Persisted refreshed Gmail access token for user {account.user_id}"
+                    )
+
             # Update run
             run.emails_forwarded = emails_forwarded  # type: ignore[assignment]
             run.emails_failed = emails_failed  # type: ignore[assignment]
@@ -210,15 +289,11 @@ async def process_all_enabled_accounts():
     """
     async with async_session_maker() as db:
         try:
-            # Get all enabled accounts
+            # Fetch all enabled accounts regardless of operational status so
+            # that accounts in ERROR state are retried automatically.
             result = await db.execute(
                 select(MailAccount).where(
-                    and_(
-                        MailAccount.is_enabled == True,  # noqa: E712
-                        MailAccount.status.in_(
-                            [AccountStatus.ACTIVE, AccountStatus.TESTING]
-                        ),
-                    )
+                    MailAccount.is_enabled == True,  # noqa: E712
                 )
             )
             accounts = result.scalars().all()
@@ -248,10 +323,10 @@ async def process_all_enabled_accounts():
 @celery_app.task(base=AsyncTask, name="app.workers.tasks.cleanup_old_logs")
 async def cleanup_old_logs(days_to_keep: int = 30):
     """
-    Clean up old processing logs and runs.
+    Clean up old processing logs, runs, and downloaded message ID records.
 
     Args:
-        days_to_keep: Number of days of logs to retain
+        days_to_keep: Number of days of data to retain
     """
     async with async_session_maker() as db:
         try:
@@ -275,6 +350,14 @@ async def cleanup_old_logs(days_to_keep: int = 30):
             for log in old_logs:
                 await db.delete(log)
 
+            # Delete old downloaded message ID records so the table doesn't
+            # grow unboundedly for accounts that never delete messages.
+            await db.execute(
+                delete(DownloadedMessageId).where(
+                    DownloadedMessageId.downloaded_at < cutoff_date
+                )
+            )
+
             await db.commit()
 
             logger.info(
diff --git a/docs/TODO.md b/docs/TODO.md
index 23e2604..a0679b8 100644
--- a/docs/TODO.md
+++ b/docs/TODO.md
@@ -179,6 +179,10 @@ Comprehensive task breakdown for repository improvements and production readines
 ### Not Started 📋
 - [ ] Implement Stripe webhook handling
 - [ ] Add scheduled Celery tasks for email processing
+- [x] Account enable/disable toggle (UX + backend)
+- [x] Per-user SMTP configuration (UX + backend)
+- [x] Gmail API one-click OAuth grant flow with token refresh and revocation handling
+- [x] Message deduplication (POP3 UIDL + IMAP \Seen flag + DB tracking)
 - [ ] Implement GDPR data export endpoint
 - [ ] Complete notification service integration (Apprise)
 - [ ] Add advanced email filtering
@@ -207,12 +211,13 @@ because the API client layer is missing.
 - [x] Registration page
 - [x] OAuth callback handler
 - [x] Dashboard with stats cards and processing runs table
-- [x] Mail accounts list with CRUD operations
-- [x] Settings page
-- [x] `AddMailAccountModal` component (auto-detect, test connection, all required fields)
+- [x] Mail accounts list with CRUD operations + enable/disable toggle
+- [x] Settings page — Profile, Gmail API connection, SMTP relay, Account info, Security
+- [x] `AddMailAccountModal` component (auto-detect, test connection, all required fields, is_enabled checkbox)
 - [x] `DashboardLayout` with responsive sidebar
 - [x] `AuthGuard` for protected routes
 - [x] Fix wizard grey screen (Tailwind v4 `bg-opacity` → `/75` syntax, modal restructure)
+- [x] `/auth/gmail-callback` page for Gmail OAuth one-click flow
 
 ### Not Started 📋
 - [ ] End-to-end testing of frontend against backend API
diff --git a/frontend/src/app/accounts/page.tsx b/frontend/src/app/accounts/page.tsx
index 0a69fe8..35e08e5 100644
--- a/frontend/src/app/accounts/page.tsx
+++ b/frontend/src/app/accounts/page.tsx
@@ -4,7 +4,7 @@ import { AuthGuard } from '@/components/AuthGuard';
 import { DashboardLayout } from '@/components/DashboardLayout';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { mailAccountsApi, MailAccount } from '@/lib/api';
-import { Plus, Edit2, Trash2, CheckCircle, XCircle, AlertTriangle } from 'lucide-react';
+import { Plus, Edit2, Trash2, CheckCircle, XCircle, AlertTriangle, Power } from 'lucide-react';
 import { useState } from 'react';
 import { AddMailAccountModal } from '@/components/AddMailAccountModal';
 
@@ -25,6 +25,13 @@ export default function AccountsPage() {
     },
   });
 
+  const toggleMutation = useMutation({
+    mutationFn: mailAccountsApi.toggle,
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['mail-accounts'] });
+    },
+  });
+
   const handleEdit = (account: MailAccount) => {
     setEditingAccount(account);
     setIsModalOpen(true);
@@ -40,6 +47,14 @@ export default function AccountsPage() {
     }
   };
 
+  const handleToggle = async (id: number) => {
+    try {
+      await toggleMutation.mutateAsync(id);
+    } catch {
+      alert('Failed to update account');
+    }
+  };
+
   const handleCloseModal = () => {
     setIsModalOpen(false);
     setEditingAccount(null);
@@ -69,7 +84,9 @@ export default function AccountsPage() {
               {accounts.map((account) => (
                 <div
                   key={account.id}
-                  className="bg-white rounded-lg shadow-md border border-gray-200 overflow-hidden"
+                  className={`bg-white rounded-lg shadow-md border overflow-hidden transition-opacity ${
+                    account.is_enabled ? 'border-gray-200' : 'border-gray-200 opacity-60'
+                  }`}
                 >
                   <div className="p-6">
                     <div className="flex items-start justify-between mb-4">
@@ -81,9 +98,15 @@ export default function AccountsPage() {
                       </div>
                       <div className="flex items-center gap-2">
                         {account.is_enabled ? (
-                          <CheckCircle className="h-5 w-5 text-green-500" aria-label="Enabled" />
+                          <span className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-xs font-medium bg-green-100 text-green-700">
+                            <CheckCircle className="h-3 w-3" />
+                            Enabled
+                          </span>
                         ) : (
-                          <XCircle className="h-5 w-5 text-gray-400" aria-label="Disabled" />
+                          <span className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-xs font-medium bg-gray-100 text-gray-500">
+                            <XCircle className="h-3 w-3" />
+                            Disabled
+                          </span>
                         )}
                       </div>
                     </div>
@@ -129,6 +152,18 @@ export default function AccountsPage() {
                     )}
 
                     <div className="flex items-center gap-2 pt-4 border-t border-gray-200">
+                      <button
+                        onClick={() => handleToggle(account.id)}
+                        disabled={toggleMutation.isPending}
+                        title={account.is_enabled ? 'Disable account' : 'Enable account'}
+                        className={`flex items-center justify-center px-3 py-2 text-sm font-medium rounded-md transition-colors disabled:opacity-50 ${
+                          account.is_enabled
+                            ? 'text-yellow-600 bg-yellow-50 hover:bg-yellow-100'
+                            : 'text-green-600 bg-green-50 hover:bg-green-100'
+                        }`}
+                      >
+                        <Power className="h-4 w-4" />
+                      </button>
                       <button
                         onClick={() => handleEdit(account)}
                         className="flex-1 flex items-center justify-center px-3 py-2 text-sm font-medium text-blue-600 bg-blue-50 rounded-md hover:bg-blue-100 transition-colors"
diff --git a/frontend/src/app/auth/gmail-callback/page.tsx b/frontend/src/app/auth/gmail-callback/page.tsx
new file mode 100644
index 0000000..923eed5
--- /dev/null
+++ b/frontend/src/app/auth/gmail-callback/page.tsx
@@ -0,0 +1,102 @@
+'use client';
+
+import { useEffect, useState, Suspense } from 'react';
+import { useRouter, useSearchParams } from 'next/navigation';
+import { gmailApi } from '@/lib/api';
+import { CheckCircle, XCircle, Loader2 } from 'lucide-react';
+
+function GmailCallbackContent() {
+  const router = useRouter();
+  const searchParams = useSearchParams();
+  const [status, setStatus] = useState<'loading' | 'success' | 'error'>('loading');
+  const [message, setMessage] = useState('Connecting your Gmail account…');
+
+  useEffect(() => {
+    async function handleCallback() {
+      const code = searchParams.get('code');
+      const error = searchParams.get('error');
+
+      if (error) {
+        setStatus('error');
+        setMessage(
+          error === 'access_denied'
+            ? 'You declined the Gmail permission request. No changes were made.'
+            : `Google returned an error: ${error}`
+        );
+        return;
+      }
+
+      if (!code) {
+        setStatus('error');
+        setMessage('No authorization code received from Google.');
+        return;
+      }
+
+      const redirectUri = `${window.location.origin}/auth/gmail-callback`;
+
+      try {
+        await gmailApi.saveCallback(code, redirectUri);
+        setStatus('success');
+        setMessage('Gmail connected successfully! Redirecting to settings…');
+        setTimeout(() => router.push('/settings'), 2000);
+      } catch (err: unknown) {
+        const detail =
+          err instanceof Error && 'response' in err
+            ? (err as { response?: { data?: { detail?: string } } }).response?.data
+                ?.detail
+            : null;
+        setStatus('error');
+        setMessage(detail || 'Failed to connect Gmail. Please try again.');
+      }
+    }
+
+    handleCallback();
+  }, [searchParams, router]);
+
+  return (
+    <div className="min-h-screen flex items-center justify-center bg-gray-50">
+      <div className="bg-white rounded-lg shadow-md p-8 max-w-md w-full text-center">
+        {status === 'loading' && (
+          <>
+            <Loader2 className="h-12 w-12 text-blue-500 animate-spin mx-auto mb-4" />
+            <p className="text-gray-700">{message}</p>
+          </>
+        )}
+        {status === 'success' && (
+          <>
+            <CheckCircle className="h-12 w-12 text-green-500 mx-auto mb-4" />
+            <h2 className="text-xl font-semibold text-gray-900 mb-2">Connected!</h2>
+            <p className="text-gray-600">{message}</p>
+          </>
+        )}
+        {status === 'error' && (
+          <>
+            <XCircle className="h-12 w-12 text-red-500 mx-auto mb-4" />
+            <h2 className="text-xl font-semibold text-gray-900 mb-2">Connection failed</h2>
+            <p className="text-gray-600 mb-6">{message}</p>
+            <button
+              onClick={() => router.push('/settings')}
+              className="px-4 py-2 bg-blue-600 text-white rounded-md hover:bg-blue-700"
+            >
+              Back to Settings
+            </button>
+          </>
+        )}
+      </div>
+    </div>
+  );
+}
+
+export default function GmailCallbackPage() {
+  return (
+    <Suspense
+      fallback={
+        <div className="min-h-screen flex items-center justify-center">
+          <Loader2 className="h-8 w-8 animate-spin text-blue-600" />
+        </div>
+      }
+    >
+      <GmailCallbackContent />
+    </Suspense>
+  );
+}
diff --git a/frontend/src/app/settings/page.tsx b/frontend/src/app/settings/page.tsx
index f273ef0..13fc949 100644
--- a/frontend/src/app/settings/page.tsx
+++ b/frontend/src/app/settings/page.tsx
@@ -4,9 +4,18 @@ import { useState } from 'react';
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 import { AuthGuard } from '@/components/AuthGuard';
 import { DashboardLayout } from '@/components/DashboardLayout';
-import { userApi } from '@/lib/api';
+import { userApi, gmailApi, smtpApi } from '@/lib/api';
 import { useAuthStore } from '@/store/authStore';
-import { CheckCircle, Loader2, User, Mail, Shield } from 'lucide-react';
+import {
+  CheckCircle,
+  Loader2,
+  User,
+  Mail,
+  Shield,
+  Server,
+  AlertTriangle,
+  XCircle,
+} from 'lucide-react';
 
 export default function SettingsPage() {
   return (
@@ -28,6 +37,16 @@ function SettingsContent() {
   });
   const [profileSaved, setProfileSaved] = useState(false);
 
+  // SMTP form state
+  const [smtpForm, setSmtpForm] = useState({
+    host: 'smtp.gmail.com',
+    port: 587,
+    username: '',
+    password: '',
+    use_tls: true,
+  });
+  const [smtpSaved, setSmtpSaved] = useState(false);
+
   // Refresh user data from the server
   const { data: currentUser } = useQuery({
     queryKey: ['current-user'],
@@ -35,6 +54,38 @@ function SettingsContent() {
     initialData: user ?? undefined,
   });
 
+  // Gmail credential status
+  const {
+    data: gmailCredential,
+    isLoading: gmailLoading,
+    error: gmailError,
+  } = useQuery({
+    queryKey: ['gmail-credential'],
+    queryFn: gmailApi.getCredential,
+    retry: false,
+  });
+
+  // SMTP config
+  const { data: smtpConfig, isLoading: smtpLoading } = useQuery({
+    queryKey: ['smtp-config'],
+    queryFn: smtpApi.get,
+    retry: false,
+  });
+
+  // Pre-populate SMTP form when data loads
+  const [smtpFormPopulated, setSmtpFormPopulated] = useState(false);
+  if (smtpConfig && !smtpFormPopulated) {
+    setSmtpFormPopulated(true);
+    setSmtpForm((prev) => ({
+      ...prev,
+      host: smtpConfig.host,
+      port: smtpConfig.port,
+      username: smtpConfig.username,
+      use_tls: smtpConfig.use_tls,
+      password: '', // never pre-fill password
+    }));
+  }
+
   const updateProfileMutation = useMutation({
     mutationFn: (data: { full_name: string; email: string }) =>
       userApi.updateProfile(data),
@@ -46,11 +97,45 @@ function SettingsContent() {
     },
   });
 
+  const disconnectGmailMutation = useMutation({
+    mutationFn: gmailApi.disconnect,
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['gmail-credential'] });
+    },
+  });
+
+  const saveSmtpMutation = useMutation({
+    mutationFn: smtpApi.save,
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['smtp-config'] });
+      setSmtpSaved(true);
+      setTimeout(() => setSmtpSaved(false), 3000);
+    },
+  });
+
+  const deleteSmtpMutation = useMutation({
+    mutationFn: smtpApi.remove,
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['smtp-config'] });
+      setSmtpForm({ host: 'smtp.gmail.com', port: 587, username: '', password: '', use_tls: true });
+    },
+  });
+
   const handleProfileChange = (e: React.ChangeEvent<HTMLInputElement>) => {
     const { name, value } = e.target;
     setProfileForm((prev) => ({ ...prev, [name]: value }));
   };
 
+  const handleSmtpChange = (e: React.ChangeEvent<HTMLInputElement | HTMLSelectElement>) => {
+    const { name, value, type } = e.target;
+    setSmtpForm((prev) => ({
+      ...prev,
+      [name]: type === 'checkbox' ? (e.target as HTMLInputElement).checked
+              : name === 'port' ? Number(value)
+              : value,
+    }));
+  };
+
   const handleProfileSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     try {
@@ -68,7 +153,55 @@ function SettingsContent() {
     }
   };
 
+  const handleSmtpSubmit = async (e: React.FormEvent) => {
+    e.preventDefault();
+    try {
+      await saveSmtpMutation.mutateAsync({
+        host: smtpForm.host,
+        port: smtpForm.port,
+        username: smtpForm.username,
+        password: smtpForm.password || undefined,
+        use_tls: smtpForm.use_tls,
+      });
+    } catch (error) {
+      const errorMessage =
+        error instanceof Error && 'response' in error
+          ? (error as { response?: { data?: { detail?: string } } }).response?.data
+              ?.detail
+          : null;
+      alert(errorMessage || 'Failed to save SMTP settings');
+    }
+  };
+
+  const handleConnectGmail = async () => {
+    try {
+      const redirectUri = `${window.location.origin}/auth/gmail-callback`;
+      const url = await gmailApi.getAuthorizeUrl(redirectUri);
+      window.location.href = url;
+    } catch (error) {
+      const errorMessage =
+        error instanceof Error && 'response' in error
+          ? (error as { response?: { data?: { detail?: string } } }).response?.data
+              ?.detail
+          : null;
+      alert(errorMessage || 'Failed to start Gmail authorization');
+    }
+  };
+
+  const handleDisconnectGmail = async () => {
+    if (confirm('Disconnect Gmail? Mail accounts using Gmail API delivery will fall back to SMTP.')) {
+      try {
+        await disconnectGmailMutation.mutateAsync();
+      } catch {
+        alert('Failed to disconnect Gmail');
+      }
+    }
+  };
+
   const displayUser = currentUser ?? user;
+  const gmailConnected = gmailCredential?.is_valid === true;
+  // gmail 404 just means "not connected yet" — not a real error
+  const gmailNotConnected = !gmailCredential && !gmailLoading;
 
   return (
     <div className="space-y-8">
@@ -134,6 +267,218 @@ function SettingsContent() {
         </form>
       </div>
 
+      {/* Gmail API Section */}
+      <div className="bg-white rounded-lg shadow">
+        <div className="px-6 py-4 border-b border-gray-200 flex items-center gap-2">
+          <Mail className="h-5 w-5 text-gray-500" />
+          <h2 className="text-lg font-semibold text-gray-900">Gmail API Delivery</h2>
+        </div>
+        <div className="px-6 py-6">
+          <p className="text-sm text-gray-600 mb-4">
+            Grant this app permission to inject emails directly into your Gmail inbox.
+            This is the preferred delivery method — emails arrive with original headers
+            intact, bypassing SMTP entirely.
+          </p>
+          <p className="text-xs text-gray-500 mb-6">
+            <strong>Token lifetime:</strong> Access tokens expire after 1 hour and are
+            refreshed automatically. Refresh tokens do not expire unless you revoke
+            access via your{' '}
+            <a
+              href="https://myaccount.google.com/permissions"
+              target="_blank"
+              rel="noopener noreferrer"
+              className="text-blue-600 underline"
+            >
+              Google Account permissions
+            </a>
+            . If revoked, click &ldquo;Connect Gmail&rdquo; again to re-authorise.
+          </p>
+
+          {gmailLoading && (
+            <div className="flex items-center gap-2 text-sm text-gray-500">
+              <Loader2 className="h-4 w-4 animate-spin" />
+              Checking Gmail connection…
+            </div>
+          )}
+
+          {!gmailLoading && gmailConnected && (
+            <div className="flex items-center justify-between flex-wrap gap-4">
+              <div className="flex items-center gap-2">
+                <CheckCircle className="h-5 w-5 text-green-500" />
+                <div>
+                  <p className="text-sm font-medium text-gray-900">
+                    Connected as <span className="font-semibold">{gmailCredential.gmail_email}</span>
+                  </p>
+                  {gmailCredential.last_verified_at && (
+                    <p className="text-xs text-gray-500">
+                      Last verified: {new Date(gmailCredential.last_verified_at).toLocaleString()}
+                    </p>
+                  )}
+                </div>
+              </div>
+              <div className="flex gap-2">
+                <button
+                  onClick={handleConnectGmail}
+                  className="px-4 py-2 text-sm bg-blue-50 text-blue-700 rounded-md hover:bg-blue-100 transition-colors"
+                >
+                  Re-authorise
+                </button>
+                <button
+                  onClick={handleDisconnectGmail}
+                  disabled={disconnectGmailMutation.isPending}
+                  className="px-4 py-2 text-sm bg-red-50 text-red-700 rounded-md hover:bg-red-100 transition-colors disabled:opacity-50"
+                >
+                  Disconnect
+                </button>
+              </div>
+            </div>
+          )}
+
+          {!gmailLoading && gmailCredential && !gmailCredential.is_valid && (
+            <div className="flex items-start gap-3 p-3 bg-red-50 border border-red-200 rounded-md mb-4">
+              <AlertTriangle className="h-5 w-5 text-red-500 mt-0.5 flex-shrink-0" />
+              <div className="text-sm text-red-700">
+                <strong>Gmail access was revoked.</strong> Click &ldquo;Re-authorise&rdquo; below to restore Gmail API delivery.
+              </div>
+            </div>
+          )}
+
+          {!gmailLoading && (gmailNotConnected || (gmailCredential && !gmailCredential.is_valid)) && (
+            <button
+              onClick={handleConnectGmail}
+              className="flex items-center gap-2 px-4 py-2 bg-blue-600 text-white rounded-md hover:bg-blue-700 transition-colors"
+            >
+              <Mail className="h-4 w-4" />
+              {gmailError ? 'Connect Gmail' : 'Re-authorise Gmail'}
+            </button>
+          )}
+
+          {!gmailLoading && gmailNotConnected && (
+            <div className="mt-3 flex items-center gap-2 text-sm text-gray-500">
+              <XCircle className="h-4 w-4 text-gray-400" />
+              No Gmail account connected yet.
+            </div>
+          )}
+        </div>
+      </div>
+
+      {/* SMTP Fallback Section */}
+      <div className="bg-white rounded-lg shadow">
+        <div className="px-6 py-4 border-b border-gray-200 flex items-center gap-2">
+          <Server className="h-5 w-5 text-gray-500" />
+          <h2 className="text-lg font-semibold text-gray-900">SMTP Fallback</h2>
+        </div>
+        <div className="px-6 py-6">
+          <p className="text-sm text-gray-600 mb-6">
+            Used when Gmail API is not connected or when a mail account is configured
+            to use SMTP delivery. Your credentials are stored encrypted.
+          </p>
+
+          {smtpLoading ? (
+            <div className="flex items-center gap-2 text-sm text-gray-500">
+              <Loader2 className="h-4 w-4 animate-spin" />
+              Loading SMTP settings…
+            </div>
+          ) : (
+            <form onSubmit={handleSmtpSubmit} className="space-y-4 max-w-lg">
+              <div className="grid grid-cols-3 gap-4">
+                <div className="col-span-2">
+                  <label className="block text-sm font-medium text-gray-700 mb-1">SMTP Host</label>
+                  <input
+                    type="text"
+                    name="host"
+                    value={smtpForm.host}
+                    onChange={handleSmtpChange}
+                    className="w-full px-3 py-2 border border-gray-300 rounded-md focus:outline-none focus:ring-2 focus:ring-blue-500"
+                    placeholder="smtp.gmail.com"
+                  />
+                </div>
+                <div>
+                  <label className="block text-sm font-medium text-gray-700 mb-1">Port</label>
+                  <input
+                    type="number"
+                    name="port"
+                    value={smtpForm.port}
+                    onChange={handleSmtpChange}
+                    min={1}
+                    max={65535}
+                    className="w-full px-3 py-2 border border-gray-300 rounded-md focus:outline-none focus:ring-2 focus:ring-blue-500"
+                  />
+                </div>
+              </div>
+
+              <div>
+                <label className="block text-sm font-medium text-gray-700 mb-1">Username</label>
+                <input
+                  type="text"
+                  name="username"
+                  value={smtpForm.username}
+                  onChange={handleSmtpChange}
+                  className="w-full px-3 py-2 border border-gray-300 rounded-md focus:outline-none focus:ring-2 focus:ring-blue-500"
+                  placeholder="you@example.com"
+                />
+              </div>
+
+              <div>
+                <label className="block text-sm font-medium text-gray-700 mb-1">Password</label>
+                <input
+                  type="password"
+                  name="password"
+                  value={smtpForm.password}
+                  onChange={handleSmtpChange}
+                  className="w-full px-3 py-2 border border-gray-300 rounded-md focus:outline-none focus:ring-2 focus:ring-blue-500"
+                  placeholder={smtpConfig?.has_password ? '••••••••  (leave blank to keep current)' : 'App password or SMTP password'}
+                />
+              </div>
+
+              <div className="flex items-center gap-2">
+                <input
+                  type="checkbox"
+                  id="smtp_use_tls"
+                  name="use_tls"
+                  checked={smtpForm.use_tls}
+                  onChange={handleSmtpChange}
+                  className="h-4 w-4 text-blue-600 border-gray-300 rounded"
+                />
+                <label htmlFor="smtp_use_tls" className="text-sm text-gray-700">
+                  Use STARTTLS (recommended for port 587)
+                </label>
+              </div>
+
+              <div className="flex items-center gap-3 pt-2">
+                <button
+                  type="submit"
+                  disabled={saveSmtpMutation.isPending}
+                  className="flex items-center px-4 py-2 bg-blue-600 text-white rounded-md hover:bg-blue-700 disabled:opacity-50 transition-colors"
+                >
+                  {saveSmtpMutation.isPending ? (
+                    <><Loader2 className="h-4 w-4 mr-2 animate-spin" />Saving…</>
+                  ) : (
+                    'Save SMTP Settings'
+                  )}
+                </button>
+                {smtpConfig && (
+                  <button
+                    type="button"
+                    onClick={() => deleteSmtpMutation.mutate()}
+                    disabled={deleteSmtpMutation.isPending}
+                    className="px-4 py-2 text-sm text-red-600 bg-red-50 rounded-md hover:bg-red-100 disabled:opacity-50 transition-colors"
+                  >
+                    Remove
+                  </button>
+                )}
+                {smtpSaved && (
+                  <span className="flex items-center text-sm text-green-600">
+                    <CheckCircle className="h-4 w-4 mr-1" />
+                    Saved
+                  </span>
+                )}
+              </div>
+            </form>
+          )}
+        </div>
+      </div>
+
       {/* Account Information */}
       <div className="bg-white rounded-lg shadow">
         <div className="px-6 py-4 border-b border-gray-200 flex items-center gap-2">
@@ -204,3 +549,4 @@ function SettingsContent() {
     </div>
   );
 }
+
diff --git a/frontend/src/components/AddMailAccountModal.tsx b/frontend/src/components/AddMailAccountModal.tsx
index 75c2c72..8b3c834 100644
--- a/frontend/src/components/AddMailAccountModal.tsx
+++ b/frontend/src/components/AddMailAccountModal.tsx
@@ -315,6 +315,19 @@ export function AddMailAccountModal({ account, onClose }: AddMailAccountModalPro
                         Delete after forwarding
                       </label>
                     </div>
+                    <div className="flex items-center">
+                      <input
+                        type="checkbox"
+                        name="is_enabled"
+                        id="is_enabled"
+                        checked={formData.is_enabled ?? true}
+                        onChange={handleChange}
+                        className="h-4 w-4 text-blue-600 focus:ring-blue-500 border-gray-300 rounded"
+                      />
+                      <label htmlFor="is_enabled" className="ml-2 block text-sm text-gray-700">
+                        Enabled
+                      </label>
+                    </div>
                   </div>
 
                   <div>
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index 68b913f..4c64f2d 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -116,6 +116,28 @@ export interface AutoDetectSuggestion {
   [key: string]: unknown;
 }
 
+export interface GmailCredential {
+  id: number;
+  user_id: number;
+  gmail_email: string;
+  is_valid: boolean;
+  last_verified_at?: string | null;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface UserSmtpConfig {
+  id: number;
+  user_id: number;
+  host: string;
+  port: number;
+  username: string;
+  use_tls: boolean;
+  has_password: boolean;
+  created_at: string;
+  updated_at: string;
+}
+
 interface TokenResponse {
   access_token: string;
   refresh_token: string;
@@ -204,6 +226,11 @@ export const mailAccountsApi = {
     return response.data;
   },
 
+  async toggle(id: number): Promise<MailAccount> {
+    const response = await api.patch<MailAccount>(`/mail-accounts/${id}/toggle`);
+    return response.data;
+  },
+
   async delete(id: number): Promise<void> {
     await api.delete(`/mail-accounts/${id}`);
   },
@@ -245,4 +272,61 @@ export const processingRunsApi = {
   },
 };
 
+// ── Gmail API ───────────────────────────────────────────────────────────
+
+export const gmailApi = {
+  /** Returns the Google OAuth2 URL the user should be redirected to. */
+  async getAuthorizeUrl(redirectUri: string): Promise<string> {
+    const response = await api.get<{ authorization_url: string }>(
+      '/providers/gmail/authorize-url',
+      { params: { redirect_uri: redirectUri } }
+    );
+    return response.data.authorization_url;
+  },
+
+  /** Exchange an OAuth2 code for Gmail tokens and persist them. */
+  async saveCallback(code: string, redirectUri: string): Promise<GmailCredential> {
+    const response = await api.post<GmailCredential>('/providers/gmail/callback', {
+      code,
+      redirect_uri: redirectUri,
+    });
+    return response.data;
+  },
+
+  /** Get the current user's stored Gmail credential status. */
+  async getCredential(): Promise<GmailCredential> {
+    const response = await api.get<GmailCredential>('/providers/gmail-credential');
+    return response.data;
+  },
+
+  /** Remove stored Gmail credentials. */
+  async disconnect(): Promise<void> {
+    await api.delete('/providers/gmail-credential');
+  },
+};
+
+// ── SMTP Config API ─────────────────────────────────────────────────────
+
+export const smtpApi = {
+  async get(): Promise<UserSmtpConfig> {
+    const response = await api.get<UserSmtpConfig>('/users/smtp-config');
+    return response.data;
+  },
+
+  async save(data: {
+    host: string;
+    port: number;
+    username: string;
+    password?: string;
+    use_tls: boolean;
+  }): Promise<UserSmtpConfig> {
+    const response = await api.put<UserSmtpConfig>('/users/smtp-config', data);
+    return response.data;
+  },
+
+  async remove(): Promise<void> {
+    await api.delete('/users/smtp-config');
+  },
+};
+
 export default api;