Microsoft fixes classic Outlook bug that stopped Gmail and Yahoo accounts syncing
Since 26 Feb 2026 classic Outlook failed to sync Gmail and Yahoo (0x800CCC0E/0F, 0x80070057). Microsoft fixed it on 20 March; stale OAuth tokens may linger.
From 26 February 2026, some Gmail and Yahoo accounts in classic Outlook for Windows stopped syncing without warning. Users got errors 0x800CCC0E, 0x800CCC0F or 0x80070057, and Outlook never asked them to sign in again. Microsoft fixed the problem with a Microsoft 365 service change on 20 March 2026. Machines with stale cached OAuth tokens may still need manual cleanup, and that's the part IT and desktop support teams need to know.
What happened
Microsoft's support article lists the symptoms by error code:
- 0x800CCC0E during "Synchronizing subscribed folders" and "Check for new mail".
- 0x800CCC0F and 0x80070057 when sending.
Accounts could also stop syncing after the user changed their Gmail or Yahoo password. In every case Outlook failed silently: no sign-in prompt appeared to refresh credentials. Microsoft says it resolved the root cause in the Microsoft 365 service on 20 March 2026. It also notes that after a password change the symptoms can still appear until the cached OAuth token expires, typically about an hour, after which Outlook should show a sign-in prompt.
For accounts that stay stuck, Microsoft documents two workarounds. The first removes the cached OAuth credentials from Windows Credential Manager. The second deletes the account's identity key in the registry. Microsoft says neither removes the account or its data.
Who is affected
- Users of classic Outlook for Windows (Microsoft 365 apps, Office 16.0 registry hive) with Gmail or Yahoo accounts connected over IMAP with OAuth.
- Not affected: the new Outlook for Windows, Outlook on the web, and Exchange or Microsoft 365 mailboxes.
- Help desks supporting small businesses and contractors who use consumer mail providers in Outlook.
What it means for developers and engineering teams
The developer impact is small, but the failure mode is worth noting. Classic Outlook connects to Gmail and Yahoo with IMAP and SMTP authenticated through OAuth 2.0 (XOAUTH2). It stores tokens in Windows Credential Manager under MicrosoftOffice16_Data:OAUTH2 entries. When the service-side token handling broke, the client got authentication failures but didn't treat them as a reason to prompt the user again. It kept retrying with a token that was no longer valid.
If you build clients that use OAuth refresh tokens, this is a common bug to avoid. Map an invalid_grant response, or a protocol-level auth failure, to an interactive re-auth path. Don't map it to a generic network error. Surface "sign in again" clearly, and give users a way to clear cached credentials without editing the registry.
What to do now
- Restart Outlook and, if you recently changed the mail password, wait about an hour for the old token to expire. A sign-in prompt should follow.
- Workaround 1 (Credential Manager): close Outlook, open Credential Manager → Windows Credentials, and under Generic Credentials remove the
MicrosoftOffice16_Data:OAUTH2entries whose full names containtp_google_imap_Oauth2ortp_yahoo_imap_Oauth2(two to four per account). Restart Outlook and sign in. - Match the right entries: to avoid removing other accounts' tokens, compare the numeric prefix with the subkeys under
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity\Identities. - Workaround 2 (registry): if that fails, back up the registry, close Outlook, delete the subkey under
...\Identity\Identitiesthat matches the failing email address, then restart and sign in. - For fleets, script the cleanup with PowerShell, for example
cmdkey /list | Select-String tp_google_imapto find entries andcmdkey /delete:<target>to remove them. Test on one machine first. - Consider the new Outlook or web clients for consumer-mail users if classic Outlook OAuth problems keep coming back.