my outlook won't sync

Use the following checks and fixes in order.

1. Confirm whether the issue is with Microsoft 365 Groups only

• In Outlook, check if the folders that show “This folder has not been updated yet” are Microsoft 365 group mailboxes (under “Groups”).
• If regular mailbox folders (Inbox, Sent Items) sync correctly but Groups do not, follow step 2. If nothing syncs (all folders), skip to step 3.

2. Fix Microsoft 365 Groups not synchronizing This issue is commonly caused by either disabled modern authentication or very large group folders.

A. Ensure modern authentication is enabled

• Modern authentication must be enabled in Exchange Online for Outlook to sync Microsoft 365 Groups correctly.
• After enabling modern auth (per tenant admin steps in Enable modern authentication for Outlook in Exchange Online), create a new Outlook profile and confirm Outlook is using modern auth:
  1. Create a new Outlook profile as described in Create an Outlook profile.
  2. Open the Outlook Connection Status dialog and verify the Authn column shows Bearer for the connections. This confirms modern authentication.
  3. Check the status bar for “This folder is up to date.”

B. Reduce the cached time window for Groups if they are very large If modern auth is already enabled and some Groups have 100,000+ items, reduce how much Group content Outlook caches.

Warning: Editing the registry incorrectly can cause serious problems. Back up the registry first.

1. Open Registry Editor.
2. Go to: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode (Create the Cached Mode key if it does not exist.)
3. Create a new DWORD (32-bit) Value named: GroupsSyncWindowSetting
4. Set its value (Decimal) to 3 (this means Outlook caches 3 months of Microsoft 365 Group content).
5. Close Registry Editor and restart Outlook.
6. Confirm the status bar shows “This folder is up to date.”
7. Use the synchronization monitor to repair folder sync If general folder sync is unreliable (frequent resyncs, stuck folders), enable the sync monitor and adjust its behavior.

Warning: Registry changes should be done carefully and backed up first.

1. Open Registry Editor.
2. Navigate to one of:
   • HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode
   • or HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\Cached Mode
3. Ensure the Cached Mode key exists (create it if needed).
4. Create or edit a DWORD (32-bit) Value named: SyncMonitorLevel
5. Set its value (Decimal) to 2 as a good starting point:
   • 2 = use improved detection logic for read/unread counts and item-change decisions, and allow the full repair process including integrity checks and clearing offline items when needed.
6. (Optional) To collect diagnostics, create another DWORD named SyncMonitorLogging and set it to 1. When the sync monitor detects a problem and repairs it, it logs details in the Sync Issues folder with subject Sync Monitor.
7. Restart Outlook and allow time for the sync monitor to run. It will:
   • Run a simple folder sync.
   • Fix known problems in local sync state.
   • Run an integrity check on the .ost file.
   • Clear offline items if needed, forcing a full resync from the server.

If full resyncs are causing performance issues on very large folders, set SyncMonitorLevel to 3 instead to prevent the “clear offline items” step, understanding that this may leave some folders unrepaired if they are badly out of sync.

4. If using IMAP/POP and sync suddenly stopped after updates If the account is configured as IMAP or POP and sync stopped after recent updates, TLS 1.3 on the client can sometimes interfere.

Warning: Only change TLS registry keys if comfortable and after exporting a backup.

1. Open Registry Editor and navigate to the Protocols key under the system’s TLS configuration.
2. If a TLS 1.3 subkey exists under Protocols, export it (right-click > Export) to back it up.
3. Delete the TLS 1.3 key and its subkeys.
4. Restart Outlook; if sync still fails, restart Windows.
5. If Outlook still will not sync

• After applying the above, if Outlook still does not sync any folders, consider creating a brand-new Outlook profile and letting it download everything again.
• If the problem persists across profiles and after registry-based sync repair, further investigation with Microsoft Support may be required, using the Sync Monitor diagnostic messages in the Sync Issues folder as input.

References:

• Microsoft 365 groups don't synchronize in Outlook
• Synchronization monitor in Microsoft Outlook
• Outlook IMAP or POP server unexpectedly terminated the connection, and the server was interrupted