Does https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-and-exchange-online-september-2021-update/ba-p/2772210 help?

xref https://www.reddit.com/r/Thunderbird/comments/vu0rri/microsoft365_and_pop3_issues_with_version_102/

Wayne - I don't think so. All connections are via Oauth2 so don't think that support article is relevant?

https://support.mozilla.org/en-US/kb/automatic-conversion-google-mail-accounts-oauth20 describes what was done for gmail.

AIUI, the changes for oauth at o365 are not proceeding well. Have you contacted them?

MS contacted indirectly via my organizaion help desk. So sorta but not really... Regardless, Oauth2 worked fine before upgrade to 102.0.1 for both gmail and office365. Our IT says there were no changes to office365 coincident with this POP3 connection problem.

Also, confirmed antivirus is presumably not involved - problem still occurs without AV running

I started having a similar problem where pop3 wouldn't work (and it was working earlier last week). I went thru a few things, including upgrading to TB 102 from 91. Same results, and my problem was a box popping up during username/password negotiation saying badness.

I had our IT guy check that link above for basic auth disable, and he came back and said we didn't have it disabled (running that diagnostic). But then about 10 minutes later he said "oh, I found another area that let me look at disabled protocols, and pop3 was listed in there. So I just allowed basic auth again for that. Try it". And behold, my pop3 access worked again.

I actually tried oauth2 access and it did not work for me. I guess I now have until Oct 1 to figure that part out, but as of now, my pop3 with normal password is back up and running with 365.

And sorry, I don't have the detail for where he found the settings to muck with, but hopefully others with admin access can find it.

The problem persists with TB 102.0.2

See: https://support.mozilla.org/en-US/que.../1381491