Improve IMAP reconnection and error handling in polling

Refactored PollInboxAsync to better handle IMAP connection drops and protocol errors by reconnecting and resuming processing as needed. Switched from using a using statement to explicit disposal in a finally block. Now logs and recovers from transient IMAP issues, and ensures proper cancellation handling by rethrowing OperationCanceledException. This increases service robustness and reliability.

src/SpamGuard/Services/InboxMonitorService.cs

@@ -90,62 +90,84 @@ public sealed partial class InboxMonitorService : BackgroundService
 
     private async Task PollInboxAsync(CancellationToken ct)
     {
-        using var client = await _imapFactory.CreateConnectedClientAsync(_imap, ct);
+        var client = await _imapFactory.CreateConnectedClientAsync(_imap, ct);
-        var inbox = client.Inbox;
+        try
-        await inbox.OpenAsync(FolderAccess.ReadWrite, ct);
+        {
+            var inbox = client.Inbox;
+            await inbox.OpenAsync(FolderAccess.ReadWrite, ct);
 
-        // Build search query: only fetch new messages
+            // Build search query: only fetch new messages
-        IList<UniqueId> uids;
+            IList<UniqueId> uids;
-        if (_lastSeenUid > 0)
+            if (_lastSeenUid > 0)
-        {
-            var range = new UniqueIdRange(new UniqueId(_lastSeenUid + 1), UniqueId.MaxValue);
-            uids = await inbox.SearchAsync(range, SearchQuery.All, ct);
-        }
-        else if (_processedUids.Count > 0)
-        {
-            // Resuming from persisted state -- scan recent messages only
-            uids = await inbox.SearchAsync(
-                SearchQuery.DeliveredAfter(DateTime.UtcNow.AddDays(-1)), ct);
-        }
-        else
-        {
-            // First ever run -- initial scan window
-            uids = await inbox.SearchAsync(
-                SearchQuery.DeliveredAfter(DateTime.UtcNow.AddDays(-_monitoring.InitialScanDays)), ct);
-        }
-        _logger.LogDebug("Found {Count} messages in inbox", uids.Count);
-
-        // Find the spam/junk folder
-        var spamFolder = await FindSpamFolderAsync(client, ct);
-
-        foreach (var uid in uids)
-        {
-            if (_processedUids.Contains(uid.Id))
             {
-                if (uid.Id > _lastSeenUid) _lastSeenUid = uid.Id;
+                var range = new UniqueIdRange(new UniqueId(_lastSeenUid + 1), UniqueId.MaxValue);
-                continue;
+                uids = await inbox.SearchAsync(range, SearchQuery.All, ct);
             }
-
+            else if (_processedUids.Count > 0)
-            try
             {
-                await ProcessMessageAsync(inbox, uid, spamFolder, ct);
+                // Resuming from persisted state -- scan recent messages only
+                uids = await inbox.SearchAsync(
+                    SearchQuery.DeliveredAfter(DateTime.UtcNow.AddDays(-1)), ct);
             }
-            catch (Exception ex)
+            else
             {
-                _logger.LogError(ex, "Error processing UID={Uid}", uid.Id);
+                // First ever run -- initial scan window
-                _activityLog.Add(new ActivityEntry
+                uids = await inbox.SearchAsync(
+                    SearchQuery.DeliveredAfter(DateTime.UtcNow.AddDays(-_monitoring.InitialScanDays)), ct);
+            }
+            _logger.LogDebug("Found {Count} messages in inbox", uids.Count);
+
+            // Find the spam/junk folder
+            var spamFolder = await FindSpamFolderAsync(client, ct);
+
+            foreach (var uid in uids)
+            {
+                if (_processedUids.Contains(uid.Id))
                 {
-                    Timestamp = DateTime.UtcNow, Sender = "", Subject = $"UID {uid.Id}",
+                    if (uid.Id > _lastSeenUid) _lastSeenUid = uid.Id;
-                    Verdict = Verdict.Error, Reason = ex.Message,
+                    continue;
-                    Uid = uid.Id, AccountName = AccountName
+                }
-                });
+
-                _processedUids.Add(uid.Id);
+                try
+                {
+                    await ProcessMessageAsync(inbox, uid, spamFolder, ct);
+                }
+                catch (OperationCanceledException) when (ct.IsCancellationRequested)
+                {
+                    throw;
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogError(ex, "Error processing UID={Uid}", uid.Id);
+                    _activityLog.Add(new ActivityEntry
+                    {
+                        Timestamp = DateTime.UtcNow, Sender = "", Subject = $"UID {uid.Id}",
+                        Verdict = Verdict.Error, Reason = ex.Message,
+                        Uid = uid.Id, AccountName = AccountName
+                    });
+                    _processedUids.Add(uid.Id);
+
+                    // Reconnect if the server dropped the connection (e.g. unexpected response after MoveToAsync)
+                    if (!client.IsConnected || ex is MailKit.Net.Imap.ImapProtocolException)
+                    {
+                        _logger.LogWarning("IMAP connection lost for {Account}, reconnecting", AccountName);
+                        try { client.Dispose(); } catch { }
+                        client = await _imapFactory.CreateConnectedClientAsync(_imap, ct);
+                        inbox = client.Inbox;
+                        await inbox.OpenAsync(FolderAccess.ReadWrite, ct);
+                        spamFolder = await FindSpamFolderAsync(client, ct);
+                    }
+                }
+
+                if (uid.Id > _lastSeenUid) _lastSeenUid = uid.Id;
             }
 
-            if (uid.Id > _lastSeenUid) _lastSeenUid = uid.Id;
+            await client.DisconnectAsync(true, ct);
+        }
+        finally
+        {
+            client.Dispose();
         }
-
-        await client.DisconnectAsync(true, ct);
     }
 
     private async Task ProcessMessageAsync(