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

@@ -90,7 +90,9 @@ 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);
+        try
+        {
             var inbox = client.Inbox;
             await inbox.OpenAsync(FolderAccess.ReadWrite, ct);
 
@@ -130,6 +132,10 @@ public sealed partial class InboxMonitorService : BackgroundService
                 {
                     await ProcessMessageAsync(inbox, uid, spamFolder, ct);
                 }
+                catch (OperationCanceledException) when (ct.IsCancellationRequested)
+                {
+                    throw;
+                }
                 catch (Exception ex)
                 {
                     _logger.LogError(ex, "Error processing UID={Uid}", uid.Id);
@@ -140,6 +146,17 @@ public sealed partial class InboxMonitorService : BackgroundService
                         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;
@@ -147,6 +164,11 @@ public sealed partial class InboxMonitorService : BackgroundService
 
             await client.DisconnectAsync(true, ct);
         }
+        finally
+        {
+            client.Dispose();
+        }
+    }
 
     private async Task ProcessMessageAsync(
         IMailFolder inbox, UniqueId uid, IMailFolder? spamFolder, CancellationToken ct)