Outlook Desktop (latest) stopped syncing IMAP accpounts from selfowned Mail server.

Sep 17, 2022
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At a certain day about 2 weeks ago Outlook 365 on a Windows 11 PC stopped synchronizing certain IMAP accounts from our own email servers. Yahoo, Gmail IMAP working fine.

This error comes at each Send/Receive operation in Outlook.
"Task '(e-mail address removed): Folder:Inbox Check for new mail.' reported error (0x800CCC0E) : 'Outlook cannot download folder Inbox from the IMAP email server for account (e-mail address removed). Error: Cannot connect to the server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'"

Windows Mail app on the same PC is synchronizing the IMAP accounts fine.
Outlook on other PCs, iPad, Android keep working fine with these IMAP accounts.
Webmailer of these IMAP accounts ok.
Nothing on our Mail server changed and as other outlook installations keep on working fine I don't think it's a server sided problem.
User credentials and settings in email accounts settings of windows did not change and have been double-checked several times.

The problem seems to be NOT from Outlook because when I use the Mail for Outlook Profile tool under the Windows Control Panel to setup an IMAP account from the same server it gives me alerts and won't let me save the account:
Error message at Test Account Settings... button clicked: "Log onto incoming mail server (IMAP): The server rejected your logon. Verify that your user name and password are correct."
By the way: SMTP/Outgoing server test is working - test message is sent each time.

Here's what we tried without success:
- changed passwords of the IMAP accounts on the server and the email account settings on the PC. Avoided special characters in the new passwords.
- double-check incoming and outgoing Server name, Ports, SSL/TLS set as required.
- Tried other ports and connection encryption (None/STARTTLS/Auto)
- extended Server Timeout time to 5 minutes
- no certficate errors
- IMAP account deleted in Control Panel > Mail for Microsoft Outlook > Setup Email Accounts and directions and setup the IMAP account anew.
- Creating a completely new IMAP account on the Mailserver and in Windows
- Creating a completely new Outlook Profile and setup the failing IMAP accounts in that new profile.
- OST-File deleted, a new one will be created automatically at Outlook start
- Setup these IMAP accounts in Windows Mail - YES, Windows Mail syncs without a problem.
- Setup the same accounts as POP3 works but that's not what we can use for a longer time.
- deactivated Windows internal Firewall
- no Antivirus/Malware service other then Windows internal installed.
- latest office update
- latest Windows 11 update

Would be thankful for any more hints what we could try - I would rather prefer to not install Windows from Scratch as long as there are other things to try.
Is there a way to just re-install the Mail for Microsoft Outlook setup tool?
Maybe something in the registry like deleting all settings for those IMAP accounts in order to setup them from scratch?

Thanks for reading.


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