Thunderbird IMAP responded: User is authenticated but not connected

My issue solved by disabling IPv6 from windows network.

Since yesterday, my Thunderbird client cannot connect to the outlook.office365.com IMAP server and other users at my university are also having this problem. I have deleted the stored token and redone the connection procedure but it does not work. I get two error messages:

The current operation on 'inbox' did not succeed. The mail server for account ... (IMAP) responded: User is authenticated but not connected.

Server outlook.office365.com has disconnected. The server may have gone down or there may be a network problem.

This does not work on any platform: Windows, Linux or Macos. Via the browser, we can access the emails but it is a workaround because it does not support openPGP. We need Thunderbird

Hey Juan

This is taking place as a result of Microsoft turning on Modern Authentication and enabled security defaults, both of which forbid earlier client apps, such as Office 2013, from being maintained because they now demand modern authentication. Office 2016 is also no longer supported.

In order to utilize Thunderbird, you must connect into Azure and disable the security defaults if you are the organization's IT administrator and oversee Office 365. Disable Modern authentication as well.

Additionally, make sure that SMTP authentication is enabled and turned on for each and every mailbox by visiting their account.

Additionally, Thunderbird won't function if MFA is configured for users because that calls for the new MS Outlook App.

You may easily install the new Outlook if your company uses M365 Apps for Enterprise by heading to Office Portaland selecting "install office."

If this is helpful, let me know.

I have same problem with Windows 10 workstation using Thunderbird and try to load IMAP messages (oauth2). POP3 does not work at all, cannot even login. SMTP works fine. Same account works on Mac thunderbird fine. Web Outlook works also fine.