PSA: We're aware of an issue affecting #Thunderbird users using #IMAP to connect to certain #Microsoft account servers (for example: outlook.office365.com).

These users are receiving an error message that says "User is authenticated but not connected."

We believe this is caused by Microsoft running certain services with broken #IPv6 functionality.

WORKAROUND: Disable IPv6 networking in Thunderbird's advanced config.

MORE INFO: https://answers.microsoft.com/en-us/outlook_com/forum/all/thunderbird-imap-responded-user-is-authenticated/062a82f6-e678-4462-88b7-dd6cc318386f?page=1