microsoft has resolved a problem that was causing synchronization problems between classic Outlook and Gmail or Yahoo accounts, according to an update in the company’s support documentation. The affected problem microsoft 365 users and started February 26, 2026. The bug prevented affected accounts from syncing and generated error codes. 0x800CCC0F and 0x80070057.
Outlook sync issue resolved, but delays may persist
Microsoft reports that the issue has been resolved in the Microsoft 365 service as of Friday. However, some users may still experience temporary issues due to the way authentication tokens behave.
The company notes that Outlook may not prompt users to sign back in immediately after the fix. Synchronization will resume once the OAuth token expires, which typically occurs about an hour after a password change.
How to restore classic Outlook sync faster
Users who prefer not to wait for the token to expire can manually trigger a sign-in prompt using a workaround provided by Microsoft.
To do this, open Windows Registry Editor, Navigate to the relevant location, delete the registry entries for the affected email account, then restart Outlook and sign in again when prompted. Microsoft shared this solution before implementing the service-side solution.
Other classic Outlook problems that Microsoft is investigating
Microsoft is investigating additional issues affecting the classic Outlook client. An error can cause errors when indicating “I can’t connect to the server” when creating groups in environments where Exchange Web Services are enabled.
Another issue affects Microsoft 365 apps, including Outlook and OneNote, where some users report that the mouse pointer disappears. Microsoft has encouraged affected customers to submit diagnostic logs through a Microsoft 365 administrative support case to assist with ongoing investigations.
What to do if Outlook Classic won’t sync Gmail or Yahoo
Users experiencing sync issues should wait for their OAuth token to expire or change their password to initiate a new login. If the sync still does not resume, the registry workaround can be used to force reauthentication.
When the classic Outlook sync fix comes to Microsoft 365 users
The fix has already been implemented in the Microsoft 365 service. Microsoft notes, however, that the full resolution depends on the expiration of the OAuth token and may take up to an hour to affect affected users.
