Microsoft fixes bug causing Classic Outlook sync issues with Gmail

A recently resolved compatibility issue in the classic Outlook client has cleared a recurring roadblock for users relying on Gmail and Yahoo accounts. The problem manifested as failed synchronization between these mail services and the older Outlook client, surfaced with specific error codes, and left affected users with a silent halt to their email flow.

The trouble began when attempts to sync Gmail and Yahoo accounts produced 0x800CCC0F and 0x80070057 errors. In the period surrounding February 26, 2026, these accounts stopped syncing entirely, and users stopped receiving sign-in prompts, which made the disruption harder to diagnose and remediate. Microsoft acknowledged the bug and started public guidance to help admins understand and mitigate the impact while a longer-term fix was prepared.

An update to the situation indicated that the issue was resolved in the Microsoft 365 service on a Friday in March 2026. Specifically, the fix was deployed on March 20, 2026, but Microsoft cautioned that some customers might still see intermittent sync issues for a while longer. The reason for these lingering problems is tied to OAuth tokens: even after the service-side fix, tokens can continue to block sign-in until they expire. In general, if a password change occurred, the OAuth token typically expires within about one hour, at which point Outlook would prompt for sign-in again. For those unable to wait, Microsoft outlined workarounds to force a sign-in prompt and reestablish connectivity.

One notable workaround involved a registry edit in Windows. For users who could not wait for token expiry, Microsoft had previously suggested deleting the affected email address identities under the Identities key in the Windows Registry:
ComputerHKEYCURRENTUSERSoftwareMicrosoftOffice16.0CommonIdentityIdentities. This action resets stored identity data for Outlook and can help force a fresh authentication session. As with many registry changes, this should be done with care, and it’s wise to back up the registry beforehand.

Beyond the Gmail and Yahoo sync fix, Microsoft researchers are looking into other related issues. A separate bug has been reported that triggers “Can’t connect to the server” errors when creating groups in classic Outlook if Exchange Web Services (EWS) is enabled for the tenant. While not the same problem, it underscores the broader challenge of keeping the classic Outlook experience stable as backend services evolve.

Another lingering symptom noted by users affects the user interface: in some environments, the mouse pointer can disappear in classic Outlook, OneNote, and other Microsoft 365 apps. While this does not block email delivery by itself, it can detract from usability and productivity, and Microsoft has asked affected customers to submit diagnostic logs to help the Outlook Support Team analyze and address the issue.

To assist with diagnosis and faster resolution, Microsoft encouraged affected organizations to open a support case through their Microsoft 365 admin and share diagnostic log files. This helps the support team reproduce the issue and verify when the token-related problems are fully resolved across tenant configurations.

Looking back, Microsoft had been addressing other classic Outlook issues tied to the December 2025 updates, including a problem that blocked Microsoft 365 customers from opening encrypted emails. In parallel, the company has warned that a known issue could render classic Outlook unusable for some users who enable the Teams Meeting add-in, signaling that ecosystem-wide compatibility remains a moving target as features and security models continue to evolve.

As the ecosystem continues to change, the core takeaway is that token-based authentication can create delayed recoveries in classic Outlook environments even after a backend fix is released. If users stop seeing sign-in prompts after a refresh or password change, patience for token expiry is sometimes the simplest path, while administrators can deploy the security-focused workarounds to regain access sooner. The broader message is clear: while fixes roll out, individual user experiences may vary until all dependent tokens settle and clients reauthenticate cleanly.