fix: IPv4 preference + DNS cache fallback for stable POP3/IMAP connections

- Add DNS_CACHE_FALLBACK_ENABLED setting (default: true)
- Add in-process DNS cache (_dns_cache, _dns_cache_lock, _get/_set_cached_ipv4)
- Add _resolve_ipv4 (async) and _resolve_ipv4_sync: AF_INET lookup, cache on
  success, return cached IP on EAI_AGAIN when fallback enabled
- Add _IMAP4SSLwithSNI: connects to pre-resolved IPv4 but uses original
  hostname for TLS SNI/cert verification
- Add _POP3SSLWithIPv4Pref / _POP3WithIPv4Pref: override _create_socket to
  use pre-resolved IPv4 while keeping original host for POP3_SSL SNI
- Add _make_pop3_conn factory and _make_imap_client async factory
- Wire IPv4/cache helpers into all 6 connection call-sites (IMAP x3, POP3 x3)
- Distinguish transient DNS (EAI_AGAIN) from permanent in _format_connection_error
- Replace asyncio.get_event_loop() with get_running_loop() in 4 async sites
- All 75 existing unit tests pass unchanged

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/be0a3bbd-1af1-408e-b918-84753723154b

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

CHANGELOG.md

@@ -30,6 +30,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- **IPv4 preference for all POP3 and IMAP connections**: The app now resolves
+  every mail-server hostname to an IPv4 address before connecting.  This
+  prevents `ENETUNREACH` / `Network is unreachable` errors and IMAP/POP3
+  connection timeouts that occur on Docker hosts where IPv6 traffic is not
+  routed to the internet but dual-stack DNS returns AAAA records first.
+
+- **In-process DNS cache with automatic fallback** (`DNS_CACHE_FALLBACK_ENABLED`,
+  enabled by default): The last successfully resolved IPv4 address for each
+  mail-server hostname is stored in memory.  When a subsequent DNS lookup fails
+  with `EAI_AGAIN` (Temporary failure in name resolution) the cached address is
+  used instead, keeping mail delivery alive through transient resolver outages.
+  Set `DNS_CACHE_FALLBACK_ENABLED=false` in the environment to disable.
+
 - **Friendly error messages**: Introduced `_format_connection_error()` helper in
   `mail_processor.py` that translates raw OS/socket/SSL/POP3/IMAP exceptions into
   human-readable sentences including the host:port and actionable guidance (DNS
@@ -58,6 +71,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- **Transient DNS errors now produce a clear, accurate message**: `EAI_AGAIN`
+  errors (errno -3 – resolver temporarily unavailable) are now reported as
+  "Temporary DNS failure while resolving '…'" instead of the misleading "check
+  that the server address is correct" message that was shown for all DNS errors.
+
+- **`asyncio.get_event_loop()` replaced with `asyncio.get_running_loop()`** in
+  `_test_pop3_connection`, `_fetch_pop3_emails`, `post_process_pop3`, and
+  `forward_email`, silencing deprecation warnings in Python ≥ 3.10 and avoiding
+  potential `DeprecationWarning → RuntimeError` in a future Python release.
+
 - **Empty IMAP error messages** — `IMAP fetch error:` with a blank suffix was
   caused by `asyncio.TimeoutError` and `aioimaplib.Abort` having an empty
   `str()`.  The new `_format_connection_error()` helper always produces a