I'm troubleshooting two Microsoft 365 users who are receiving frequent "Synchronization Log" messages in the Deleted Items folder while using Classic Outlook. The issue began around the same time last week. I've already rebuilt the OST files, updated Microsoft 365 Apps, confirmed advanced logging is disabled, and tried the Outlook cleanviews switch, but the messages continue. The logs don't identify a useful folder or item, and one user receives them every few minutes. Has anyone found a reliable way to identify and resolve the underlying sync problem without forcing users onto New Outlook?
4 Answers
Switching to the newer client might avoid the visible messages, but it’s more of a workaround than a diagnosis, and users may lose features they depend on. I’d treat it as a last resort after testing the mailbox in the web client, checking problematic attachments or shared folders, and temporarily changing the cached-mail configuration.
These logs are usually diagnostic notices rather than evidence that mail is being lost, so the underlying problem may be minor. Unfortunately, moving them with a rule only hides the symptom, especially when the log doesn’t identify the failing folder or item. The useful fix is to find the object Outlook cannot synchronize.
This is often caused by one damaged or otherwise unprocessable mailbox item that Outlook keeps retrying. Check the mailbox in Outlook on the web around the time the errors started, looking for suspicious messages or attachments. If you find the item, permanently delete it and see whether the synchronization notices stop.
Since both mailboxes are in Microsoft 365, I’d also test whether a specific shared resource is involved. One user still relies on a calendar exposed through a public folder, which could be contributing to the repeated sync attempts. Temporarily disabling Cached Exchange Mode or removing that legacy calendar connection may help isolate the cause.
One of the users does rely on a public-folder calendar despite repeated recommendations to use a normal shared calendar. I’m going to test with Cached Exchange Mode disabled and see whether the messages stop.