Edit: I am now getting the error on both my TBird O365 accounts.

Same problem for me, started this morning, a GoDaddy office365 account.

Just curious--what are your two Security Settings (Connection Security and Authentication Method) for the affected accounts? (Tools/Account Settings then scroll to and click Server Settings located under the address of the account in question.

SSL/TLS and OAuth2.

After 36 minutes on hold, GoDaddy says that if I can log into Outlook Webmail, then it's a problem with TBird. I have my doubts. Because the problem expanded--initially only one account was erroring--it could be an MS backend issue. I hope it is, because then it will resolve when MS fixes it.

Working again. Looks like it was an MS backend issue.

Edit: Spoke too soon. One account is now working, the other is not.

I also noticed this problem yesterday evening - I successfully downloaded a handful of messages before getting the error message. Noticed this morning that Thunderbird had re-downloaded about 20 messages from 10 May , but nothing form today...

Still not resolved. I'll file a bug report later today.

We have had this problem too. Two Outlook accounts - one started failing about two / three days ago the other a day later. I reported to Cobweb (our hoster) and got the 'we don't support T-bird, Outlook looks as though it's fine, it must be Thunderbird' response. Yesterday I was getting inconsistent results to get mail. mostly the Stat failure. Then got start of a download of messages which was interupted by service unavailable 21. I reported this to cobweb at about midnight last night. (9th). No response from them and during the day situation looked unchanged. Having watched TV for a couple of hours this evening, finally had another go and got about 25 messages downloaded on mine and a dozen or so on my wife's. All now looks to have been sorted. Notified Cobweb just after 9pm saying I would fully believe it if it's still ok tomorrow morning!

Mine is going up and down. One very interesting data point: I have four GoDaddy/Outlook accounts on two domains. Each domain has a main account and an alias account. One of those two domain does not show the problem on either of its email accounts. The other domain is showing problem with the main account but not usually with the alias account. I decided to auto-forward the main account's email to the alias account so I would at least see the email. As soon as I set up the main account's auto-forwarding, the alias account stopped working and wouldn't work until I disabled the auto-forward. This happened twice. All that said, at this moment both accounts are working.

My Emails have successfully arrived this morning...

Mine as well. Perhaps it was an MS problem after all (not that they'd say anything about it).

I had this error message immediately after a Firefox update & reboot. Was working fine just before that. Can anyone offer a solution?

My email just came through, 6 times for each. Crazy, oh well, will never complain about that again.