Outlook Sync Issues Slowing Email

Question

Getting many sync error issues every day in Outlook synchronization Log, slows down sending and receiving email.

Answer 1

Hello John
Thank you for reaching out

I understand you're experiencing frequent Outlook synchronization errors that are slowing email send and receive performance.

Please try the following:

1. Open Outlook in Safe Mode and check if the issue persists.
2. Create a new Outlook profile and test synchronization.
3. If you're using a Microsoft 365 or Exchange account, rebuild the OST file and allow Outlook to resync.
4. Run the Inbox Repair Tool (SCANPST.EXE) to check and repair Outlook data file corruption. This can help resolve synchronization issues caused by a damaged Outlook data file.
5. Test the account in Outlook on the web. If the issue doesn't occur there, the problem is likely related to the Outlook desktop client or local profile.

For references :

• https://support.microsoft.com/en-us/outlook/repair-outlook-data-files-pst-and-ost https://learn.microsoft.com/en-us/troubleshoot/exchange/client-connectivity/ost-sync-issues

To help investigate further, please provide:

• The exact error message from the Synchronization Issues folder.
• Your Outlook version (Classic Outlook, New Outlook, or Outlook for Microsoft 365).
• Your account type (Microsoft 365, Exchange, IMAP, or POP).

Answer 2

The failing mailbox is the Comcast IMAP account—not the Outlook.com account—so first correct its Xfinity server settings and then repair the Outlook profile if error 800CCC0F-0-0-560 continues.

1. Verify the Comcast IMAP and SMTP configuration

Error 0x800CCC0F means the connection to the IMAP/POP server was interrupted or unexpectedly terminated; it does not by itself identify whether the interruption is caused by the server, network, credentials, or Outlook’s local profile.^1^

1. In classic Outlook, select File > Account Settings > Server Settings.
2. Select the Comcast account and compare its settings with Xfinity’s current configuration:^2^
   • Incoming mail
     • Server: imap.comcast.net
     • Port: 993
     • Encryption: SSL/TLS
     • Username: the complete Comcast email address
   • Outgoing mail
     • Server: smtp.comcast.net
     • Port: 465 with SSL, or 587 with STARTTLS/TLS
     • Outgoing authentication enabled, using the same account credentials
3. Save the settings and restart Outlook.
4. Select Send/Receive > Send/Receive All Folders.
5. Confirm that the Inbox finishes synchronizing without creating another 800CCC0F-0-0-560 entry.

Also open the Comcast/Xfinity mailbox in webmail. If webmail is similarly slow or unavailable, the problem is upstream of Outlook and should be reported to Xfinity.

2. Repair the Outlook profile

If the settings are already correct and webmail works normally:

1. Select File > Account Settings > Account Settings.
2. On the Email tab, select the Comcast IMAP account.
3. Select Repair and complete the wizard.
4. Restart Outlook and test Send/Receive All Folders again.^3^

If Repair is unavailable or the error remains, create a new Outlook profile and add the Comcast account to it as IMAP. A new profile forces Outlook to create a fresh local synchronization state rather than continuing to use the existing one.

3. Reduce unnecessary IMAP folder synchronization

If Outlook works but repeatedly spends time synchronizing many Comcast folders:

1. Open Send/Receive > Send/Receive Groups > Define Send/Receive Groups.
2. Select All Accounts, then Edit.
3. Select the Comcast account.
4. Clear Get folder unread count for subscribed folders.
5. Under received mail items, select Inbox only, if that option meets your needs.^4^

This reduces background folder work but should be used only if you do not require every IMAP folder to update during each automatic send/receive.

Conditional credential and Windows checks

• If Comcast requires an app password, or this began after changing the Comcast password, create a fresh app password under the provider’s webmail security settings. Then enter it through File > Account Information > select the Comcast account > Account Settings > Server Settings > Password.^1^
• Do not use the Credential Manager OAuth workaround documented for Gmail and Yahoo. The affected account here is Comcast, so that workaround’s applicability condition is not met.
• Only on Windows 10, if the safer steps fail and Registry Editor shows a manually configured TLS 1.3 key under SCHANNEL\Protocols, Microsoft documents exporting the key for backup and removing that TLS 1.3 configuration. Registry changes are risky; create the export first and skip this branch entirely on Windows 11.^1^

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References

1. Outlook IMAP or POP server unexpectedly terminated the connection, and the server was interrupted | Microsoft Support
2. Set up your Xfinity Email address with an email program
3. Fix your Outlook email connection by repairing your profile
4. Outlook folders synching not completing - Microsoft Q&A