lost sync between Gmail emails and Outlook.

Update classic Outlook, restart it, and then force a Google sign-in refresh if both Gmail accounts still fail to synchronize.

1. Check whether Gmail is receiving the messages

Open each account at Gmail.com in a browser:

• Messages appear in Gmail: delivery is working; the problem is between Outlook and Google.
• Messages do not appear in Gmail: this is not an Outlook synchronization problem. Check Gmail filters, forwarding, storage, and the sender’s delivery status.

No broad Gmail/classic Outlook outage is currently documented, while Outlook–Google authentication problems have previously caused this symptom ^1^.

2. Update and restart Outlook

In classic Outlook:

1. Select File → Office Account.
2. Select Update Options → Update Now.
3. Install all available updates.
4. Close and reopen Outlook.
5. Select Send/Receive → Send/Receive All Folders.

Microsoft tracks fixes and workarounds for supported classic Outlook versions—including Microsoft 365, Outlook 2024, 2021, 2019, and 2016—on its current known-issues page ^2^.

3. If Outlook reports 0x800CCC0E, 0x800CCC0F, or 0x80070057

These errors can occur when Outlook does not prompt for the Google sign-in needed to refresh its OAuth credentials. Refresh them separately for each affected Gmail account ^3^:

1. Close Outlook.
2. Open Windows Start, search for Credential Manager, and open it.
3. Select Windows Credentials.
4. Under Generic Credentials, expand entries beginning with: MicrosoftOffice16_Data:OAUTH2
5. Identify entries whose full names contain: tp_google_imap_Oauth2
6. Remove the matching Google OAuth entries. There may be two or four per Gmail account.
7. Restart Outlook.
8. Sign in to Google when prompted and grant Outlook the requested access.
9. If synchronization does not begin, restart Outlook once more.

Removing these credentials does not remove the Gmail accounts or their email; it causes Outlook to request fresh Google authentication.

4. Check Gmail’s IMAP settings only if Outlook still cannot connect

For each Gmail account:

1. Open Gmail in a browser.
2. Select Settings → See all settings → Forwarding and POP/IMAP.
3. If the IMAP access setting is available, ensure IMAP is enabled.
4. Open the Labels tab.
5. Ensure Show in IMAP is selected for Inbox, Sent Mail, Trash, and All Mail.
6. Return to Outlook and allow up to 15 minutes for synchronization.

Microsoft documents the required Gmail connection settings as imap.gmail.com, SSL, port 993; outgoing mail uses smtp.gmail.com, authentication, and either SSL port 465 or TLS/STARTTLS port 587 ^4^.

5. Registry fallback

Use this only if deleting the OAuth credentials does not produce a Google sign-in prompt. Deleting the wrong registry key can damage another Office identity, so export the key first.

1. Close Outlook.
2. Press Windows+R, enter regedit, and select OK.
3. Navigate to: Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity\Identities
4. Inspect the subkeys and locate the identity containing the affected Gmail address.
5. Export that specific subkey as a backup.
6. Delete only that Gmail identity subkey.
7. Restart Outlook and complete the Google sign-in.

This procedure is Microsoft’s documented fallback for an Outlook profile that remains tied to stale Google credentials ^3^.

References

1. Outlook has stopped syncing with my gmail - Microsoft Q&A
2. Fixes or workarounds for recent issues in classic Outlook for Windows ...
3. Users get errors 0x800CCC0E & 0x800CCC0F synchronizing Gmail and Yahoo accounts in classic Outlook | Microsoft Support
4. Fix problems connecting your Google account to the Microsoft Cloud