diff --git a/CHANGELOG.md b/CHANGELOG.md
index f9c24f5..4dc619f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `.safety-policy.yml` to document and suppress the two unfixable `ecdsa` side-channel CVEs (64396, 64459) that the upstream maintainers have acknowledged cannot be resolved in pure Python.
 ### Fixed
 
+- **Add mail account wizard showed `[object Object]`** — FastAPI validation
+  errors return `detail` as an array of objects, not a plain string.  The
+  frontend error handler now inspects the type of `detail`: if it is a string
+  it is used directly; if it is an array the individual `msg` fields are joined;
+  otherwise the generic `Error.message` or a fallback string is shown.  The fix
+  is applied to both the Save and Test-Connection error paths.
+
 - **IMAP: fix all IMAP emails appearing empty** — `aioimaplib` stores RFC822
   literal data as `bytearray`, not `bytes`.  The FETCH extraction loop was
   checking `isinstance(line, bytes)` which returns `False` for `bytearray`,
diff --git a/frontend/src/components/AddMailAccountModal.tsx b/frontend/src/components/AddMailAccountModal.tsx
index f4382b9..01a2211 100644
--- a/frontend/src/components/AddMailAccountModal.tsx
+++ b/frontend/src/components/AddMailAccountModal.tsx
@@ -116,6 +116,18 @@ export function AddMailAccountModal({ account, onClose }: AddMailAccountModalPro
     }
   };
 
+  const extractErrorMessage = (error: unknown, fallback: string): string => {
+    interface FastAPIValidationError { msg: string; loc?: unknown[]; type?: string }
+    const detail = (error as { response?: { data?: { detail?: unknown } } })?.response?.data?.detail;
+    if (typeof detail === 'string') return detail;
+    if (Array.isArray(detail)) {
+      const msgs = (detail as FastAPIValidationError[]).map((d) => d?.msg ?? JSON.stringify(d));
+      return msgs.join('; ');
+    }
+    if (error instanceof Error && error.message) return error.message;
+    return fallback;
+  };
+
   const handleTestConnection = async () => {
     setTestStatus('testing');
     setTestMessage('');
@@ -150,10 +162,7 @@ export function AddMailAccountModal({ account, onClose }: AddMailAccountModalPro
       }
     } catch (error) {
       setTestStatus('error');
-      const errorMessage = error instanceof Error && 'response' in error
-        ? (error as { response?: { data?: { detail?: string } } }).response?.data?.detail
-        : null;
-      setTestMessage(errorMessage || 'Connection failed');
+      setTestMessage(extractErrorMessage(error, 'Connection failed'));
     }
   };
 
@@ -188,10 +197,7 @@ export function AddMailAccountModal({ account, onClose }: AddMailAccountModalPro
         await createMutation.mutateAsync(formData);
       }
     } 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 account');
+      alert(extractErrorMessage(error, 'Failed to save account'));
     }
   };