Generic 404 handler for synchronizers.

Wino.Core.Domain/MenuItems/MenuItemCollection.cs

@@ -188,6 +188,35 @@ public class MenuItemCollection : ObservableRangeCollection<IMenuItem>
         Insert(insertIndex, accountMenuItem);
     }
 
+    public bool RemoveFolderMenuItem(Guid folderId)
+    {
+        // Check root-level items.
+        var rootItem = this.OfType<IBaseFolderMenuItem>()
+            .FirstOrDefault(a => a.HandlingFolders.Any(b => b.Id == folderId));
+
+        if (rootItem != null)
+        {
+            Remove(rootItem);
+            return true;
+        }
+
+        // Check sub-items of root folders.
+        foreach (var rootFolder in this.OfType<IBaseFolderMenuItem>())
+        {
+            var subItem = rootFolder.SubMenuItems
+                .OfType<IBaseFolderMenuItem>()
+                .FirstOrDefault(a => a.HandlingFolders.Any(b => b.Id == folderId));
+
+            if (subItem != null)
+            {
+                rootFolder.SubMenuItems.Remove(subItem);
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     private void ClearFolderAreaMenuItems()
     {
         var itemsToRemove = this.Where(a => !_preservingTypesForFolderArea.Contains(a.GetType())).ToList();

Wino.Core/CoreContainerSetup.cs

@@ -4,6 +4,7 @@ using Wino.Authentication;
 using Wino.Core.Domain.Interfaces;
 using Wino.Core.Integration.Processors;
 using Wino.Core.Services;
+using Wino.Core.Synchronizers.Errors;
 using Wino.Core.Synchronizers.Errors.Gmail;
 using Wino.Core.Synchronizers.Errors.Imap;
 using Wino.Core.Synchronizers.Errors.Outlook;
@@ -49,6 +50,8 @@ public static class CoreContainerSetup
         services.AddTransient<GmailQuotaExceededHandler>();
         services.AddTransient<GmailRateLimitHandler>();
         services.AddTransient<GmailHistoryExpiredHandler>();
+        // Register shared error handlers
+        services.AddTransient<EntityNotFoundHandler>();
 
         // Register IMAP error handlers
         services.AddTransient<ImapConnectionLostHandler>();

Wino.Core/Requests/Folder/DeleteFolderRequest.cs

@@ -1,11 +1,17 @@
+using CommunityToolkit.Mvvm.Messaging;
 using Wino.Core.Domain.Entities.Mail;
 using Wino.Core.Domain.Enums;
 using Wino.Core.Domain.Models.Requests;
+using Wino.Messaging.UI;
 
 namespace Wino.Core.Requests.Folder;
 
 public record DeleteFolderRequest(MailItemFolder Folder) : FolderRequestBase(Folder, FolderSynchronizerOperation.DeleteFolder)
 {
-    public override void ApplyUIChanges() { }
+    public override void ApplyUIChanges()
+    {
+        WeakReferenceMessenger.Default.Send(new FolderDeleted(Folder));
+    }
+
     public override void RevertUIChanges() { }
 }

Wino.Core/Services/GmailSynchronizerErrorHandlingFactory.cs

@@ -1,4 +1,5 @@
 using Wino.Core.Domain.Interfaces;
+using Wino.Core.Synchronizers.Errors;
 using Wino.Core.Synchronizers.Errors.Gmail;
 
 namespace Wino.Core.Services;
@@ -12,11 +13,13 @@ public class GmailSynchronizerErrorHandlingFactory : SynchronizerErrorHandlingFa
     public GmailSynchronizerErrorHandlingFactory(
         GmailQuotaExceededHandler quotaExceededHandler,
         GmailRateLimitHandler rateLimitHandler,
-        GmailHistoryExpiredHandler historyExpiredHandler)
+        GmailHistoryExpiredHandler historyExpiredHandler,
+        EntityNotFoundHandler entityNotFoundHandler)
     {
         // Order matters - more specific handlers should be registered first
         RegisterHandler(quotaExceededHandler);
         RegisterHandler(historyExpiredHandler);
+        RegisterHandler(entityNotFoundHandler);
         RegisterHandler(rateLimitHandler); // Most generic rate limit handler last
     }
 }

Wino.Core/Services/ImapSynchronizerErrorHandlingFactory.cs

@@ -1,4 +1,5 @@
 using Wino.Core.Domain.Interfaces;
+using Wino.Core.Synchronizers.Errors;
 using Wino.Core.Synchronizers.Errors.Imap;
 
 namespace Wino.Core.Services;
@@ -12,11 +13,13 @@ public class ImapSynchronizerErrorHandlingFactory : SynchronizerErrorHandlingFac
     public ImapSynchronizerErrorHandlingFactory(
         ImapConnectionLostHandler connectionLostHandler,
         ImapAuthenticationFailedHandler authFailedHandler,
+        EntityNotFoundHandler entityNotFoundHandler,
         ImapFolderNotFoundHandler folderNotFoundHandler,
         ImapProtocolErrorHandler protocolErrorHandler)
     {
         // Order matters - more specific handlers should be registered first
         RegisterHandler(authFailedHandler);
+        RegisterHandler(entityNotFoundHandler);
         RegisterHandler(folderNotFoundHandler);
         RegisterHandler(connectionLostHandler);
         RegisterHandler(protocolErrorHandler); // Most generic, registered last

Wino.Core/Services/OutlookSynchronizerErrorHandlingFactory.cs

@@ -1,6 +1,5 @@
-using System.Threading.Tasks;
-using Wino.Core.Domain.Interfaces;
-using Wino.Core.Domain.Models.Synchronization;
+using Wino.Core.Domain.Interfaces;
+using Wino.Core.Synchronizers.Errors;
 using Wino.Core.Synchronizers.Errors.Outlook;
 
 namespace Wino.Core.Services;
@@ -8,13 +7,11 @@ namespace Wino.Core.Services;
 public class OutlookSynchronizerErrorHandlingFactory : SynchronizerErrorHandlingFactory, IOutlookSynchronizerErrorHandlerFactory
 {
     public OutlookSynchronizerErrorHandlingFactory(ObjectCannotBeDeletedHandler objectCannotBeDeleted,
+                                                 EntityNotFoundHandler entityNotFoundHandler,
                                                  DeltaTokenExpiredHandler deltaTokenExpiredHandler)
     {
         RegisterHandler(objectCannotBeDeleted);
+        RegisterHandler(entityNotFoundHandler);
         RegisterHandler(deltaTokenExpiredHandler);
     }
-
-    public bool CanHandle(SynchronizerErrorContext error) => CanHandle(error);
-
-    public Task HandleAsync(SynchronizerErrorContext error) => HandleErrorAsync(error);
 }

Wino.Core/Services/SynchronizationManager.cs

@@ -405,6 +405,11 @@ public class SynchronizationManager : ISynchronizationManager
             await synchronizer.DownloadMissingMimeMessageAsync(mailItem, null, cancellationToken);
             return mailItem.Id.ToString(); // Return some identifier, actual implementation might be different
         }
+        catch (SynchronizerEntityNotFoundException)
+        {
+            _logger.Warning("MIME message for mail item {MailItemId} no longer exists on server. Removed locally.", mailItem.Id);
+            return null;
+        }
         catch (Exception ex)
         {
             _logger.Error(ex, "Failed to download MIME message for mail item {MailItemId}", mailItem.Id);

Wino.Core/Synchronizers/Errors/EntityNotFoundHandler.cs

@@ -0,0 +1,89 @@
+using System;
+using System.Threading.Tasks;
+using Serilog;
+using Wino.Core.Domain.Enums;
+using Wino.Core.Domain.Interfaces;
+using Wino.Core.Domain.Models.Synchronization;
+
+namespace Wino.Core.Synchronizers.Errors;
+
+/// <summary>
+/// Generic handler for 404 (Not Found) errors across all synchronizers.
+/// When a resource is already gone on the server, this handler applies
+/// the intended change locally instead of throwing.
+/// Works for all mail actions, folder actions, and batch operations.
+/// </summary>
+public class EntityNotFoundHandler : ISynchronizerErrorHandler
+{
+    private readonly ILogger _logger = Log.ForContext<EntityNotFoundHandler>();
+    private readonly IMailService _mailService;
+    private readonly IFolderService _folderService;
+
+    public EntityNotFoundHandler(IMailService mailService, IFolderService folderService)
+    {
+        _mailService = mailService;
+        _folderService = folderService;
+    }
+
+    public bool CanHandle(SynchronizerErrorContext error)
+    {
+        if (error.ErrorCode != 404) return false;
+        if (error.RequestBundle == null) return false;
+        return true;
+    }
+
+    public async Task<bool> HandleAsync(SynchronizerErrorContext error)
+    {
+        error.Severity = SynchronizerErrorSeverity.Recoverable;
+        error.Category = SynchronizerErrorCategory.ResourceNotFound;
+
+        var uiRequest = error.RequestBundle.UIChangeRequest;
+
+        // --- Folder actions ---
+        if (uiRequest is IFolderActionRequest folderAction)
+        {
+            _logger.Warning("Entity not found (404) for folder operation {Op} on {RemoteFolderId}. Deleting locally.",
+                folderAction.Operation, folderAction.Folder.RemoteFolderId);
+
+            try
+            {
+                await _folderService.DeleteFolderAsync(
+                    folderAction.Folder.MailAccountId,
+                    folderAction.Folder.RemoteFolderId).ConfigureAwait(false);
+            }
+            catch (Exception ex)
+            {
+                _logger.Error(ex, "Failed to delete folder locally after 404.");
+            }
+
+            return true;
+        }
+
+        // --- Individual mail actions ---
+        if (uiRequest is IMailActionRequest mailAction && error.Account != null)
+        {
+            _logger.Warning("Entity not found (404) for mail operation {Op} on {MailId}. Deleting locally.",
+                mailAction.Operation, mailAction.Item.Id);
+
+            // Revert optimistic UI change (e.g. mark-read/flag toggle) before deleting
+            error.RequestBundle.UIChangeRequest?.RevertUIChanges();
+
+            try
+            {
+                await _mailService.DeleteMailAsync(
+                    error.Account.Id, mailAction.Item.Id).ConfigureAwait(false);
+            }
+            catch (Exception ex)
+            {
+                _logger.Error(ex, "Failed to delete mail locally after 404.");
+            }
+
+            return true;
+        }
+
+        // --- Batch requests (can't identify specific item) ---
+        // Mark as recoverable. Next sync will clean up stale items.
+        _logger.Warning("Entity not found (404) for batch operation. Marking as recoverable.");
+        return true;
+    }
+}

Wino.Core/Synchronizers/GmailSynchronizer.cs

@@ -1318,19 +1318,28 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
                                                            ITransferProgress transferProgress = null,
                                                            CancellationToken cancellationToken = default)
     {
-        var request = _gmailService.Users.Messages.Get("me", mailItem.Id);
-        request.Format = UsersResource.MessagesResource.GetRequest.FormatEnum.Raw;
-
-        var gmailMessage = await request.ExecuteAsync(cancellationToken).ConfigureAwait(false);
-        var mimeMessage = gmailMessage.GetGmailMimeMessage();
-
-        if (mimeMessage == null)
+        try
         {
-            _logger.Warning("Tried to download Gmail Raw Mime with {Id} id and server responded without a data.", mailItem.Id);
-            return;
-        }
+            var request = _gmailService.Users.Messages.Get("me", mailItem.Id);
+            request.Format = UsersResource.MessagesResource.GetRequest.FormatEnum.Raw;
 
-        await _gmailChangeProcessor.SaveMimeFileAsync(mailItem.FileId, mimeMessage, Account.Id).ConfigureAwait(false);
+            var gmailMessage = await request.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+            var mimeMessage = gmailMessage.GetGmailMimeMessage();
+
+            if (mimeMessage == null)
+            {
+                _logger.Warning("Tried to download Gmail Raw Mime with {Id} id and server responded without a data.", mailItem.Id);
+                return;
+            }
+
+            await _gmailChangeProcessor.SaveMimeFileAsync(mailItem.FileId, mimeMessage, Account.Id).ConfigureAwait(false);
+        }
+        catch (GoogleApiException ex) when (ex.HttpStatusCode == System.Net.HttpStatusCode.NotFound)
+        {
+            _logger.Warning("Gmail message {MailId} not found (404) during MIME download. Deleting locally.", mailItem.Id);
+            await _gmailChangeProcessor.DeleteMailAsync(Account.Id, mailItem.Id).ConfigureAwait(false);
+            throw new SynchronizerEntityNotFoundException(ex.Message);
+        }
     }
 
     public override async Task DownloadCalendarAttachmentAsync(
@@ -1472,12 +1481,12 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
         // Create error context
         var errorContext = new SynchronizerErrorContext
         {
+            Account = Account,
             ErrorCode = error.Code,
             ErrorMessage = error.Message,
             RequestBundle = bundle,
             AdditionalData = new Dictionary<string, object>
             {
-                { "Account", Account },
                 { "Error", error }
             }
         };

Wino.Core/Synchronizers/ImapSynchronizer.cs

@@ -235,18 +235,36 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
         var remoteFolderId = folder.RemoteFolderId;
 
         var client = await _clientPool.GetClientAsync().ConfigureAwait(false);
-        var remoteFolder = await client.GetFolderAsync(remoteFolderId, cancellationToken).ConfigureAwait(false);
 
-        var uniqueId = new UniqueId(MailkitClientExtensions.ResolveUid(mailItem.Id));
+        try
+        {
+            var remoteFolder = await client.GetFolderAsync(remoteFolderId, cancellationToken).ConfigureAwait(false);
 
-        await remoteFolder.OpenAsync(FolderAccess.ReadOnly, cancellationToken).ConfigureAwait(false);
+            var uniqueId = new UniqueId(MailkitClientExtensions.ResolveUid(mailItem.Id));
 
-        var message = await remoteFolder.GetMessageAsync(uniqueId, cancellationToken, transferProgress).ConfigureAwait(false);
+            await remoteFolder.OpenAsync(FolderAccess.ReadOnly, cancellationToken).ConfigureAwait(false);
 
-        await _imapChangeProcessor.SaveMimeFileAsync(mailItem.FileId, message, Account.Id).ConfigureAwait(false);
-        await remoteFolder.CloseAsync(false, cancellationToken).ConfigureAwait(false);
+            var message = await remoteFolder.GetMessageAsync(uniqueId, cancellationToken, transferProgress).ConfigureAwait(false);
 
-        _clientPool.Release(client);
+            await _imapChangeProcessor.SaveMimeFileAsync(mailItem.FileId, message, Account.Id).ConfigureAwait(false);
+            await remoteFolder.CloseAsync(false, cancellationToken).ConfigureAwait(false);
+        }
+        catch (FolderNotFoundException ex)
+        {
+            _logger.Warning("IMAP folder {FolderId} not found during MIME download for {MailId}. Deleting locally.", remoteFolderId, mailItem.Id);
+            await _imapChangeProcessor.DeleteMailAsync(Account.Id, mailItem.Id).ConfigureAwait(false);
+            throw new SynchronizerEntityNotFoundException(ex.Message);
+        }
+        catch (ImapCommandException ex) when (ex.Response == ImapCommandResponse.No)
+        {
+            _logger.Warning("IMAP message {MailId} not found during MIME download (NO response). Deleting locally.", mailItem.Id);
+            await _imapChangeProcessor.DeleteMailAsync(Account.Id, mailItem.Id).ConfigureAwait(false);
+            throw new SynchronizerEntityNotFoundException(ex.Message);
+        }
+        finally
+        {
+            _clientPool.Release(client);
+        }
     }
 
     public override Task DownloadCalendarAttachmentAsync(
@@ -500,16 +518,29 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
                 }
             }
 
-            // TODO: Retry pattern.
-            // TODO: Error handling.
             try
             {
                 await item.NativeRequest.IntegratorTask(executorClient, item.Request).ConfigureAwait(false);
             }
-            catch (Exception)
+            catch (Exception ex)
             {
-                item.Request.RevertUIChanges();
-                throw;
+                var errorContext = new SynchronizerErrorContext
+                {
+                    Account = Account,
+                    ErrorCode = ex is FolderNotFoundException ? 404 : null,
+                    ErrorMessage = ex.Message,
+                    Exception = ex,
+                    RequestBundle = item,
+                    OperationType = "RequestExecution"
+                };
+
+                var handled = await _errorHandlerFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);
+
+                if (!handled)
+                {
+                    item.Request.RevertUIChanges();
+                    throw;
+                }
             }
             finally
             {

Wino.Core/Synchronizers/OutlookSynchronizer.cs

@@ -1430,8 +1430,17 @@ public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message,
                                                            MailKit.ITransferProgress transferProgress = null,
                                                            CancellationToken cancellationToken = default)
     {
-        var mimeMessage = await DownloadMimeMessageAsync(mailItem.Id, cancellationToken).ConfigureAwait(false);
-        await _outlookChangeProcessor.SaveMimeFileAsync(mailItem.FileId, mimeMessage, Account.Id).ConfigureAwait(false);
+        try
+        {
+            var mimeMessage = await DownloadMimeMessageAsync(mailItem.Id, cancellationToken).ConfigureAwait(false);
+            await _outlookChangeProcessor.SaveMimeFileAsync(mailItem.FileId, mimeMessage, Account.Id).ConfigureAwait(false);
+        }
+        catch (ODataError ex) when (ex.ResponseStatusCode == 404)
+        {
+            _logger.Warning("Outlook message {MailId} not found (404) during MIME download. Deleting locally.", mailItem.Id);
+            await _outlookChangeProcessor.DeleteMailAsync(Account.Id, mailItem.Id).ConfigureAwait(false);
+            throw new SynchronizerEntityNotFoundException(ex.Message);
+        }
     }
 
     public override async Task DownloadCalendarAttachmentAsync(

Wino.Mail.ViewModels/MailAppShellViewModel.cs

@@ -988,6 +988,21 @@ public partial class MailAppShellViewModel : MailBaseViewModel,
         UpdateFolderCollection(mailItemFolder);
     }
 
+    protected override async void OnFolderDeleted(MailItemFolder folder)
+    {
+        base.OnFolderDeleted(folder);
+
+        bool wasSelected = SelectedMenuItem is IBaseFolderMenuItem selectedFolder &&
+            selectedFolder.HandlingFolders.Any(a => a.Id == folder.Id);
+
+        await ExecuteUIThread(() => MenuItems.RemoveFolderMenuItem(folder.Id));
+
+        if (wasSelected && latestSelectedAccountMenuItem != null)
+        {
+            await NavigateInboxAsync(latestSelectedAccountMenuItem);
+        }
+    }
+
     protected override void OnFolderSynchronizationEnabled(IMailItemFolder mailItemFolder)
     {
         base.OnFolderSynchronizationEnabled(mailItemFolder);

Wino.Mail.ViewModels/MailBaseViewModel.cs

@@ -17,6 +17,7 @@ public class MailBaseViewModel : CoreBaseViewModel,
     IRecipient<DraftFailed>,
     IRecipient<DraftMapped>,
     IRecipient<FolderRenamed>,
+    IRecipient<FolderDeleted>,
     IRecipient<FolderSynchronizationEnabled>
 {
     protected virtual void OnMailAdded(MailCopy addedMail) { }
@@ -27,6 +28,7 @@ public class MailBaseViewModel : CoreBaseViewModel,
     protected virtual void OnDraftFailed(MailCopy draftMail, MailAccount account) { }
     protected virtual void OnDraftMapped(string localDraftCopyId, string remoteDraftCopyId) { }
     protected virtual void OnFolderRenamed(IMailItemFolder mailItemFolder) { }
+    protected virtual void OnFolderDeleted(MailItemFolder folder) { }
     protected virtual void OnFolderSynchronizationEnabled(IMailItemFolder mailItemFolder) { }
 
     void IRecipient<MailAddedMessage>.Receive(MailAddedMessage message) => OnMailAdded(message.AddedMail);
@@ -39,6 +41,7 @@ public class MailBaseViewModel : CoreBaseViewModel,
     void IRecipient<DraftCreated>.Receive(DraftCreated message) => OnDraftCreated(message.DraftMail, message.Account);
 
     void IRecipient<FolderRenamed>.Receive(FolderRenamed message) => OnFolderRenamed(message.MailItemFolder);
+    void IRecipient<FolderDeleted>.Receive(FolderDeleted message) => OnFolderDeleted(message.MailItemFolder);
     void IRecipient<FolderSynchronizationEnabled>.Receive(FolderSynchronizationEnabled message) => OnFolderSynchronizationEnabled(message.MailItemFolder);
 
     protected override void RegisterRecipients()
@@ -55,6 +58,7 @@ public class MailBaseViewModel : CoreBaseViewModel,
         Messenger.Register<DraftFailed>(this);
         Messenger.Register<DraftMapped>(this);
         Messenger.Register<FolderRenamed>(this);
+        Messenger.Register<FolderDeleted>(this);
         Messenger.Register<FolderSynchronizationEnabled>(this);
     }
 
@@ -70,6 +74,7 @@ public class MailBaseViewModel : CoreBaseViewModel,
         Messenger.Unregister<DraftFailed>(this);
         Messenger.Unregister<DraftMapped>(this);
         Messenger.Unregister<FolderRenamed>(this);
+        Messenger.Unregister<FolderDeleted>(this);
         Messenger.Unregister<FolderSynchronizationEnabled>(this);
     }
 }

Wino.Mail.ViewModels/MailListPageViewModel.cs

@@ -780,6 +780,20 @@ public partial class MailListPageViewModel : MailBaseViewModel,
         }
     }
 
+    protected override async void OnFolderDeleted(MailItemFolder folder)
+    {
+        base.OnFolderDeleted(folder);
+
+        if (ActiveFolder == null) return;
+
+        bool isActiveFolder = ActiveFolder.HandlingFolders.Any(a => a.Id == folder.Id);
+
+        if (isActiveFolder)
+        {
+            await MailCollection.ClearAsync();
+        }
+    }
+
     protected override async void OnDraftCreated(MailCopy draftMail, MailAccount account)
     {
         base.OnDraftCreated(draftMail, account);

Wino.Messages/UI/FolderDeleted.cs

@@ -0,0 +1,5 @@
+using Wino.Core.Domain.Entities.Mail;
+
+namespace Wino.Messaging.UI;
+
+public record FolderDeleted(MailItemFolder MailItemFolder) : UIMessageBase<FolderDeleted>;