I just installed a fresh copy of Win 10, and TB 102, inside a virtual machine. No addons, defaults for everything. Set up an account to connect to my GMail - POP3, SSL/TLS, OAuth2. Worked *perfectly*, suggesting it isn't the OAuth2 implementation that has changed.

But...if I next try setting up an account for my mail on outlook.office365.com, using same TLS/SSL and OAuth2 settings (which worked perfectly using TB 91.xx), nada. No connections at all.

Huge clue --

For grins, I re-configured and used IMAP instead of POP. Worked perfectly. So, this tells me that something fundamental has changed on the MS side of things.

I loathe IMAP, and would much prefer to continue to use POP. But, something has changed (on the MS side of things?) which clobbers OAuth2 connections to the POP server on office365.

having the same basic problem (see att image), just submitted question about rolling back to 91 also emply/missing folders

running POP to AOL acct (authenticated) under Win 10

was able to connect by going into TB acct settings/server settings & selecting "Norman password" for Authentication Method

this seems to be working at present

hope this helps until 102 is fixed

Here is a summary of what I know about this 'bug' (I filed it as such with Mozilla):

1\ TB 102 - OAuth2 works perfectly for connections to outlook.office365.com, but only for IMAP connections. POP connections fail completely. If I roll TB back to 91.x.x, OAuth2 works fine for both IMAP and POP,

Obs #1: something has changed between TB 91.x.x and TB 102.y.y that breaks handshaking between TB 102.y.y and outlook.office365.com

2\ based on a suggestion posted on the Bugzilla website (copied below), I tried overriding the UserAgent in the TB config to send the string that TB 102 was in fact TB 91.

Perhaps try Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.5.1

The default can be overridden by setting the preference general.useragent.override in the config editor. See https://support.mozilla.org/en-US/kb/config-editor

The preference does not exist by default, so you will have to add a new string preference.

This did not work.

Obs #2: it isn't as simple as a UserAgent string that hasn't been 'updated' on the office365 side of things

3\ TB 102 handshakes and works fine with smtp.office365.com, via OAuth2.

Obs #3: the only issue/bug/problem seems to involve OAuth2 and POP only. Not IMAP, and not SMTP.

4\ TB 102 OAuth2 works fine with other mail hosts that support POP (eg. pop.gmail.com).

Obs #4: same as Obs #3 (above), but...only seems to be an issue with Microsoft POP.

From what I can gather, this is a 'bug' introduced in TB102. Microsoft 'claims' there are no changes on their end that would cause the problem, so we're back pointing at TB102.

Doesn't work with AOL (Verizon) POP either.

I've more or less given up on TB102 -- the fact that it is no longer on the release updates for 91.x.xis somewhat telling (i.e., the devs probably realized that the number of bugs is not worth the effort). For further empirical evidence, check out the number of 'bugs being worked on': https://bugzilla.mozilla.org/show_bug.cgi?id=1772703

Fortunately,I'm completely content with 91.x.x.

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

Thanks, but I don't think so. The link you posted is largely a discussion of when 'basic auth' is being turned off. For my institution, its already off -- we need to use OAuth2 for everything (IMAP, POP, SMTP). I've been (successfully) using OAuth2 for POP and SMTP, with TB 91.x.x. TB102 works with SMTP, and IMAP (although I don't use IMAP), but doesn't work with POP.

I conferred with Level 2 support at MS, as per my institutions arrangements with MS, and they categorically say no changes were made on 'their end' that would have cause OAuth2 to fail for TB102. Again, it worked fine (and continues to work fine) using TB 91.x.x, so the problem seems to point very much at TB102 - and some sort of regression in the 'update' from 91 -> 102.

Now a confirmed bug -- in theory, patched/fix in Thunderbird 103.0b6 - https://hg.mozilla.org/releases/comm-beta/rev/562acd4516f9

Worth a try. I do my 'software testing' in a virtual machine (rather than on my production machines), so if it works - great. If not, no real loss of anything important (like my email).

Will wonders ever cease -- problem solved with beta6 (tested in a Win 10 VM about30 seconds ago). Works with Office365, Gmail, and (I suspect) most everything else.

On the other hand, now that I've waded through this (and 3-4 other pernicious bugs), and played around with 102/103 for a bit, I see absolutely no reason to upgrade from 91.x.x. The changes (as opposed to 'enhancements') seem to be largely aesthetic. Personally, I could care less about aesthetics -- its an email client. Functionality is (to me, and I suspect most users) waaaaaaaaaaaaay more important that 'what it looks like'.

So, TB 91.x.x for me for a while (translation: so long as it works, and the few extensions I use still function).

I have the same issue with my Earthlink account. Thunderbird 102 works fine with my other email account since the update. Both are pop accounts. I tried setting the account to normal password, and SSL/TLS, which should be supported by Earthlink, but still no server connection.

If version 103 does fix this, how do I get this version so I can install it? And if it is a beta version, will it then not be as stable as 102?