Microsoft Outlook Classic Sync Failure with Gmail/Yahoo Fixed – Service Disruption Resolved
What Happened — A bug in the classic Outlook client caused synchronization errors (0x800CCC0F, 0x80070057) when connecting to Gmail or Yahoo accounts, halting email flow for affected users. Microsoft patched the issue in the Microsoft 365 service on 26 Feb 2026; remaining users may need to wait for OAuth token expiration or apply a registry‑based workaround.
Why It Matters for TPRM —
- Email is a primary communication channel for most third‑party relationships; disruption can delay contract negotiations, invoice processing, and incident reporting.
- OAuth token handling flaws expose organizations to credential‑reuse risks if tokens are not promptly revoked after password changes.
- The fix demonstrates Microsoft’s rapid response but also highlights the need for continuous monitoring of vendor‑provided SaaS tools.
Who Is Affected — Enterprises and SMBs using classic Outlook (Office 2016/2019/365) to access Gmail or Yahoo mail, across all industries.
Recommended Actions —
- Verify that all Outlook clients have received the February 2026 update; enforce automatic updates where possible.
- Instruct users to change passwords and confirm token expiration (≈1 hour) or apply the registry workaround to force re‑authentication.
- Review OAuth token lifecycle policies with Microsoft 365 admin to ensure timely revocation.
Technical Notes — The issue stemmed from an OAuth token expiration mismatch, triggering error codes 0x800CCC0F and 0x80070057 during IMAP/SMTP sync. No CVE was assigned. A temporary registry edit under HKCU\Software\Microsoft\Office\16.0\Common\Identity\Identities forces token renewal. Microsoft is also investigating related EWS group‑creation errors and UI glitches (mouse pointer disappearance). Source: BleepingComputer