Microsoft has fixed the issue that was causing the issue sync issues between classic Outlook and Gmail or Yahoo accounts, according to an update in the company’s support document. The issue affected Microsoft 365 users and started 26 February 2026. The bug prevented affected accounts from syncing and resulted in error codes 0x800CCC0F and 0x80070057.
Outlook sync issue resolved, but delays may persist
Microsoft reports that the issue on the Microsoft 365 service has been resolved as of Friday. However, some users may still experience temporary issues due to the behavior of authentication tokens.
The company says Outlook may no longer prompt users to sign in again immediately after the fix. Syncing will resume as soon as the OAuth token expires, which usually happens about an hour after changing the password.
How to restore classic Outlook syncing faster
Users who prefer not to wait for the token to expire can manually trigger the 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 if prompted. Microsoft shared this workaround before deploying the service-side fix.
Microsoft is investigating other classic Outlook problems
Microsoft is investigating additional issues affecting the classic Outlook client. A bug can cause errors in telling “can not connect to server” When creating groups in environments where Exchange Web Services is 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 the Microsoft 365 Admin Support case to help with the ongoing investigation.
What to do if Classic Outlook 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 sign-in. If syncing still does not resume, a registry workaround can be used to force re-authentication.
When the classic Outlook sync fix reaches Microsoft 365 users
This solution has already been implemented on Microsoft 365 service. However, Microsoft notes that full resolution depends on the expiration of the OAuth token and may take up to an hour to affect affected users.
