I'm supporting two Microsoft 365 users who suddenly started receiving frequent Synchronization Log messages in Outlook's Deleted Items folder. Both users are running Classic Outlook, and the issue began around the middle of last week. The messages don't provide enough detail to identify the underlying problem.
So far, I've rebuilt the OST files, updated Office, confirmed that advanced logging is disabled, and tried Outlook's clean views option, but nothing has helped. One user receives a message every few minutes, while the other also gets them regularly. One mailbox still relies on older features such as sharing calendars through a public folder.
Has anyone found a reliable way to identify and stop the synchronization failure without forcing the users to switch to New Outlook?
4 Answers
The synchronization entries are usually harmless from a mail-delivery standpoint, but Outlook can still create them as unread items in Deleted Items. Unfortunately, moving them with a rule only hides the symptom; the useful fix is finding the folder or item that Outlook cannot process.
Switching to New Outlook might avoid the visible messages, but it doesn’t explain the underlying problem and may not be practical for users who depend on Classic Outlook features. I’d first inspect the mailbox online and test cached mode settings before treating a client change as the solution.
Since both mailboxes are in Microsoft 365, try testing with Cached Exchange Mode disabled for one affected user. For users who are almost always online, cached mode can sometimes create more synchronization problems than it solves. This is especially worth testing on the mailbox that still uses older public-folder calendar sharing.
This is often caused by one corrupt or otherwise unprocessable mailbox item that Outlook keeps retrying. It may be spam or a message with a problematic attachment. Check the mailbox through Outlook on the web around the time the errors started, locate the suspicious item, and permanently delete it. That can break the retry loop.
That makes sense. One user receives the messages every few minutes, so I can test the cached-mode change and inspect the mailbox online while the issue is actively occurring.