better 404 handling.

Wino.Core.Domain/Models/Synchronization/SynchronizerErrorContext.cs

@@ -82,6 +82,13 @@ public class SynchronizerErrorContext
     /// </summary>
     public string OperationType { get; set; }
 
+    /// <summary>
+    /// Gets or sets whether the error was explicitly classified as a missing remote entity.
+    /// This is used to distinguish true "mail/folder/event no longer exists" cases from
+    /// unrelated HTTP 404 responses that should still surface to the user.
+    /// </summary>
+    public bool IsEntityNotFound { get; set; }
+
     /// <summary>
     /// Gets whether this error should be retried based on severity and retry count.
     /// </summary>

Wino.Core.Tests/Synchronizers/GmailSynchronizerRequestSuccessTests.cs

@@ -5,6 +5,7 @@ using FluentAssertions;
 using Google.Apis.Requests;
 using Moq;
 using Wino.Core.Domain.Entities.Mail;
+using Wino.Core.Domain.Exceptions;
 using Wino.Core.Domain.Entities.Shared;
 using Wino.Core.Domain.Interfaces;
 using Wino.Core.Domain.Models.Synchronization;
@@ -142,6 +143,70 @@ public sealed class GmailSynchronizerRequestSuccessTests
         errorFactory.Verify(x => x.HandleErrorAsync(It.IsAny<SynchronizerErrorContext>()), Times.Once);
     }
 
+    [Fact]
+    public async Task ProcessSingleNativeRequestResponseAsync_Generic404Error_DoesNotClassifyAsEntityNotFound()
+    {
+        var changeProcessor = new Mock<IGmailChangeProcessor>(MockBehavior.Strict);
+        SynchronizerErrorContext? capturedContext = null;
+
+        var errorFactory = new Mock<IGmailSynchronizerErrorHandlerFactory>(MockBehavior.Strict);
+        errorFactory
+            .Setup(x => x.HandleErrorAsync(It.IsAny<SynchronizerErrorContext>()))
+            .Callback<SynchronizerErrorContext>(context => capturedContext = context)
+            .ReturnsAsync(false);
+
+        var synchronizer = CreateSynchronizer(changeProcessor.Object, errorFactory.Object);
+        var request = new DeleteRequest(CreateMailCopy("mail-1"));
+        var bundle = new HttpRequestBundle<IClientServiceRequest>(Mock.Of<IClientServiceRequest>(), request, request);
+        using var response = new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new StringContent(string.Empty)
+        };
+        var error = new RequestError
+        {
+            Code = 404,
+            Message = "Not Found"
+        };
+
+        var act = () => InvokeProcessSingleNativeRequestResponseAsync(synchronizer, bundle, response, error);
+
+        await act.Should().ThrowAsync<SynchronizerException>();
+        capturedContext.Should().NotBeNull();
+        capturedContext!.IsEntityNotFound.Should().BeFalse();
+    }
+
+    [Fact]
+    public async Task ProcessSingleNativeRequestResponseAsync_Entity404Error_ClassifiesAsEntityNotFound()
+    {
+        var changeProcessor = new Mock<IGmailChangeProcessor>(MockBehavior.Strict);
+        SynchronizerErrorContext? capturedContext = null;
+
+        var errorFactory = new Mock<IGmailSynchronizerErrorHandlerFactory>(MockBehavior.Strict);
+        errorFactory
+            .Setup(x => x.HandleErrorAsync(It.IsAny<SynchronizerErrorContext>()))
+            .Callback<SynchronizerErrorContext>(context => capturedContext = context)
+            .ReturnsAsync(false);
+
+        var synchronizer = CreateSynchronizer(changeProcessor.Object, errorFactory.Object);
+        var request = new DeleteRequest(CreateMailCopy("mail-1"));
+        var bundle = new HttpRequestBundle<IClientServiceRequest>(Mock.Of<IClientServiceRequest>(), request, request);
+        using var response = new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new StringContent(string.Empty)
+        };
+        var error = new RequestError
+        {
+            Code = 404,
+            Message = "Requested entity was not found."
+        };
+
+        var act = () => InvokeProcessSingleNativeRequestResponseAsync(synchronizer, bundle, response, error);
+
+        await act.Should().ThrowAsync<SynchronizerEntityNotFoundException>();
+        capturedContext.Should().NotBeNull();
+        capturedContext!.IsEntityNotFound.Should().BeTrue();
+    }
+
     private static GmailSynchronizer CreateSynchronizer(
         IGmailChangeProcessor changeProcessor,
         IGmailSynchronizerErrorHandlerFactory? errorFactory = null)

Wino.Core/Services/GmailSynchronizerErrorHandlingFactory.cs

@@ -21,7 +21,7 @@ public class GmailSynchronizerErrorHandlingFactory : SynchronizerErrorHandlingFa
         RegisterHandler(authenticationFailedHandler);
         RegisterHandler(quotaExceededHandler);
         RegisterHandler(historyExpiredHandler);
+        RegisterHandler(rateLimitHandler);
         RegisterHandler(entityNotFoundHandler);
-        RegisterHandler(rateLimitHandler); // Most generic rate limit handler last
     }
 }

Wino.Core/Synchronizers/Errors/EntityNotFoundHandler.cs

@@ -8,26 +8,27 @@ using Wino.Core.Domain.Models.Synchronization;
 namespace Wino.Core.Synchronizers.Errors;
 
 /// <summary>
-/// Generic handler for 404 (Not Found) errors across all synchronizers.
+/// Handles errors that were explicitly classified as missing remote entities.
-/// When a resource is already gone on the server, this handler applies
+/// This avoids swallowing unrelated HTTP 404 responses that should still surface
-/// the intended change locally instead of throwing.
+/// to the user as real synchronization failures.
-/// 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;
+    private readonly ICalendarService _calendarService;
 
-    public EntityNotFoundHandler(IMailService mailService, IFolderService folderService)
+    public EntityNotFoundHandler(IMailService mailService, IFolderService folderService, ICalendarService calendarService)
     {
         _mailService = mailService;
         _folderService = folderService;
+        _calendarService = calendarService;
     }
 
     public bool CanHandle(SynchronizerErrorContext error)
     {
-        if (error.ErrorCode != 404) return false;
+        if (!error.IsEntityNotFound) return false;
         if (error.RequestBundle == null) return false;
         return true;
     }
@@ -81,9 +82,30 @@ public class EntityNotFoundHandler : ISynchronizerErrorHandler
             return true;
         }
 
+        // --- Individual calendar actions ---
+        if (uiRequest is ICalendarActionRequest calendarAction)
+        {
+            _logger.Warning("Entity not found for calendar operation {Op} on {CalendarItemId}. Deleting locally.",
+                calendarAction.Operation, calendarAction.Item?.Id);
+
+            try
+            {
+                if (calendarAction.Item != null)
+                {
+                    await _calendarService.DeleteCalendarItemAsync(calendarAction.Item.Id).ConfigureAwait(false);
+                }
+            }
+            catch (Exception ex)
+            {
+                _logger.Error(ex, "Failed to delete calendar item locally after entity-not-found.");
+            }
+
+            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.");
+        _logger.Warning("Entity not found for batch operation. Marking as recoverable.");
         return true;
     }
 }

Wino.Core/Synchronizers/GmailSynchronizer.cs

@@ -1665,6 +1665,8 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
     {
         if (error == null) return;
 
+        var isEntityNotFound = IsKnownGmailEntityNotFoundError(error, bundle);
+
         // Create error context
         var errorContext = new SynchronizerErrorContext
         {
@@ -1672,6 +1674,7 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
             ErrorCode = error.Code,
             ErrorMessage = error.Message,
             RequestBundle = bundle,
+            IsEntityNotFound = isEntityNotFound,
             AdditionalData = new Dictionary<string, object>
             {
                 { "Error", error }
@@ -1702,7 +1705,7 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
             }
 
             // Entity not found.
-            if (error.Code == 404)
+            if (isEntityNotFound)
             {
                 bundle?.UIChangeRequest?.RevertUIChanges();
                 throw new SynchronizerEntityNotFoundException(error.Message);
@@ -1718,6 +1721,50 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
         }
     }
 
+    private static bool IsKnownGmailEntityNotFoundError(
+        RequestError error,
+        IRequestBundle<IClientServiceRequest> bundle)
+    {
+        if (error?.Code != 404 || bundle?.UIChangeRequest == null)
+            return false;
+
+        if (!IsExistingEntityOperation(bundle.UIChangeRequest))
+            return false;
+
+        var message = error.Message?.Trim() ?? string.Empty;
+        if (string.IsNullOrWhiteSpace(message))
+            return false;
+
+        var normalizedMessage = message.ToLowerInvariant();
+        return normalizedMessage.Contains("requested entity")
+               || normalizedMessage.Contains("message not found")
+               || normalizedMessage.Contains("thread not found")
+               || normalizedMessage.Contains("draft not found")
+               || normalizedMessage.Contains("label not found")
+               || normalizedMessage.Contains("event not found")
+               || normalizedMessage.Contains("calendar not found");
+    }
+
+    private static bool IsExistingEntityOperation(IUIChangeRequest request)
+        => request is BatchDeleteRequest
+           || request is BatchMoveRequest
+           || request is BatchChangeFlagRequest
+           || request is BatchMarkReadRequest
+           || request is BatchArchiveRequest
+           || request is DeleteRequest
+           || request is MoveRequest
+           || request is ChangeFlagRequest
+           || request is MarkReadRequest
+           || request is ArchiveRequest
+           || request is RenameFolderRequest
+           || request is DeleteFolderRequest
+           || request is AcceptEventRequest
+           || request is DeclineEventRequest
+           || request is OutlookDeclineEventRequest
+           || request is TentativeEventRequest
+           || request is UpdateCalendarEventRequest
+           || request is DeleteCalendarEventRequest;
+
     private static bool ShouldRevertOptimisticMailStateChange(IUIChangeRequest request)
         => request is BatchMarkReadRequest
         || request is MarkReadRequest

Wino.Core/Synchronizers/ImapSynchronizer.cs

@@ -763,7 +763,8 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
                     ErrorMessage = ex.Message,
                     Exception = ex,
                     RequestBundle = item,
-                    OperationType = "RequestExecution"
+                    OperationType = "RequestExecution",
+                    IsEntityNotFound = ex is FolderNotFoundException || ex is SynchronizerEntityNotFoundException
                 };
 
                 var handled = await _errorHandlerFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);

Wino.Core/Synchronizers/OutlookSynchronizer.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Net;
 using System.Net.Http;
 using System.Text;
 using System.Text.Json;
@@ -1860,6 +1861,7 @@ public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message,
             ErrorCode = (int)response.StatusCode,
             ErrorMessage = errorMessage,
             RequestBundle = bundle,
+            IsEntityNotFound = IsKnownOutlookEntityNotFoundError(response.StatusCode, errorCode, errorMessage, bundle),
             AdditionalData = new Dictionary<string, object>
             {
                 { "ErrorCode", errorCode },
@@ -1880,6 +1882,49 @@ public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message,
         }
     }
 
+    private static bool IsKnownOutlookEntityNotFoundError(
+        HttpStatusCode statusCode,
+        string errorCode,
+        string errorMessage,
+        IRequestBundle<RequestInformation> bundle)
+    {
+        if (statusCode != HttpStatusCode.NotFound || bundle?.UIChangeRequest == null)
+            return false;
+
+        if (!IsExistingEntityOperation(bundle.UIChangeRequest))
+            return false;
+
+        var normalizedErrorCode = errorCode?.Trim().ToLowerInvariant() ?? string.Empty;
+        var normalizedMessage = errorMessage?.Trim().ToLowerInvariant() ?? string.Empty;
+
+        return normalizedErrorCode.Contains("notfound")
+               || normalizedErrorCode.Contains("itemnotfound")
+               || normalizedErrorCode.Contains("resource")
+               || normalizedMessage.Contains("not found")
+               || normalizedMessage.Contains("does not exist")
+               || normalizedMessage.Contains("cannot be found");
+    }
+
+    private static bool IsExistingEntityOperation(IUIChangeRequest request)
+        => request is BatchDeleteRequest
+           || request is BatchMoveRequest
+           || request is BatchChangeFlagRequest
+           || request is BatchMarkReadRequest
+           || request is BatchArchiveRequest
+           || request is DeleteRequest
+           || request is MoveRequest
+           || request is ChangeFlagRequest
+           || request is MarkReadRequest
+           || request is ArchiveRequest
+           || request is RenameFolderRequest
+           || request is DeleteFolderRequest
+           || request is AcceptEventRequest
+           || request is DeclineEventRequest
+           || request is OutlookDeclineEventRequest
+           || request is TentativeEventRequest
+           || request is UpdateCalendarEventRequest
+           || request is DeleteCalendarEventRequest;
+
     private async Task HandleSuccessfulResponseAsync(IRequestBundle<RequestInformation> bundle, HttpResponseMessage response)
     {
         try