Hallo Jens
Mein Problem habe ich wie folgt gelöst.
Auf einer englischsprachigen Seite von Thunderbird hat jemand eine Mail des Microsoftsupportes veröffentlicht. Microsoft empfiehlt zur Lösung des Serverproblems den IMAP oder POP Server zu ändern.
IMAP Server neu outlook.office365.com Port bleibt: 993
Postausgangsserver neu smtp.office365.com Port bleibt: 587
Auszug
After a long and sometimes frustrating email war with Microsoft support I finally got them to admit that they were having server issues. They provided a fix which I have been using now for over a month and everything is back to the good normal. Below is their solution;
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I have received some information back from our agents after they went through your issue. They would like you to use this information with the account to test if we see a change in how that is acting for you. Due to outlook.com server migration, the mail server settings are changed to Server name Port Requires SSL (TLS) Incoming Mail (IMAP) Server: outlook.office365.com 993 or 143 Yes (always encrypted) or Yes (STARTTLS) Incoming Mail (POP) Server: outlook.office365.com 995 Yes (always encrypted) Outgoing Mail (SMTP) Server: smtp.office365.com 587 Yes (STARTTLS)
Get back to me if you see any issues with that still after so we can have this looked into further