Refactored all synchronizers to deal with some of the chronic issues.

Wino.Core.Domain/Enums/SynchronizationCompletedState.cs

@@ -4,5 +4,6 @@ public enum SynchronizationCompletedState
 {
     Success, // All succeeded.
     Canceled, // Canceled by user or HTTP call.
-    Failed // Exception.
+    Failed, // Exception.
+    PartiallyCompleted // Some folders succeeded, some failed.
 }

Wino.Core.Domain/Enums/SynchronizerErrorCategory.cs

@@ -0,0 +1,47 @@
+namespace Wino.Core.Domain.Enums;
+
+/// <summary>
+/// Categorizes synchronization errors by their root cause for targeted handling.
+/// </summary>
+public enum SynchronizerErrorCategory
+{
+    /// <summary>
+    /// Network-related issues: connection timeouts, DNS failures, socket errors.
+    /// </summary>
+    Network,
+
+    /// <summary>
+    /// Authentication failures: invalid credentials, expired tokens, revoked access.
+    /// </summary>
+    Authentication,
+
+    /// <summary>
+    /// Rate limiting: too many requests (HTTP 429), quota exceeded.
+    /// </summary>
+    RateLimit,
+
+    /// <summary>
+    /// Resource not found: folder or message deleted externally (HTTP 404).
+    /// </summary>
+    ResourceNotFound,
+
+    /// <summary>
+    /// Server errors: internal server errors (HTTP 5xx), service unavailable.
+    /// </summary>
+    ServerError,
+
+    /// <summary>
+    /// Protocol errors: IMAP/SMTP command failures, malformed responses.
+    /// </summary>
+    ProtocolError,
+
+    /// <summary>
+    /// Validation errors: invalid data, constraint violations.
+    /// </summary>
+    Validation,
+
+    /// <summary>
+    /// Unknown or unclassified error.
+    /// </summary>
+    Unknown
+}

Wino.Core.Domain/Enums/SynchronizerErrorSeverity.cs

@@ -0,0 +1,31 @@
+namespace Wino.Core.Domain.Enums;
+
+/// <summary>
+/// Classifies the severity of synchronization errors to determine retry behavior.
+/// </summary>
+public enum SynchronizerErrorSeverity
+{
+    /// <summary>
+    /// Transient error that should be retried with exponential backoff.
+    /// Examples: network timeout, temporary server unavailability, rate limiting.
+    /// </summary>
+    Transient,
+
+    /// <summary>
+    /// Error that can be recovered from by skipping the affected item/folder and continuing sync.
+    /// Examples: folder deleted externally, message not found, permission denied on single item.
+    /// </summary>
+    Recoverable,
+
+    /// <summary>
+    /// Fatal error that requires stopping synchronization and user intervention.
+    /// Examples: account disabled, server permanently unavailable, critical configuration error.
+    /// </summary>
+    Fatal,
+
+    /// <summary>
+    /// Authentication error that requires the user to re-authenticate.
+    /// Examples: token expired, password changed, OAuth refresh failed.
+    /// </summary>
+    AuthRequired
+}