Hi,
i have found this article on microsoft:
https://answers.microsoft.com/en-us/outlook_com/forum/all/thunderbird-imap-responded-user-is-authenticated/062a82f6-e678-4462-88b7-dd6cc318386f?page=2
For thunderbird they are pointing to disable ipv6.
My pc with thunderbird only has ipv6 and is working fine, but my zammad server is using ipv6 so maybe there may be some truth in this?