ERROR: Mail server outlook.office365.com responded: Server Unavailable. 21
Hello. I have been asked by Microsoft Support to contact Thunderbird to resolve a recurrent error in the POP connection. At unpredictable times, Thunderbird stops downloading messages with error message Server Unavailable 21. Microsoft finds their systems run properly and they think it is a Thunderbird problem. The error is also described on Reddit by other users.
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Perhaps you could provide a link to the support topic with Microsoft.
This is from Microsoft Enterprise support (translated):
"Okay, in that case, we'll have to check with Thunderbird support. For our part, we haven't noticed any problems with our POP connections to your account. Access on OWA is OK, and it will be the same if you were using Outlook thick client.
Given that you're using a 3rd party app, you'll need to check with its support for this synchronization problem, noting that everything's fine with Microsoft. Since there are several reports of this kind on the net, there must have been a blockage in one of their updates, with the date of the 17th."
and "So, if the problem recurs, check whether there's an incident on the administration portal. If not, contact us by creating a ticket, and in parallel contact Mozilla support to save time. In this kind of management case, if it's not an incident on our platforms, it's obviously a problem with the 3rd party software (Mozilla here).
Symptom(s): POP / IMAP connection problem with Thunderbird for Mac OS"