The 'too many messages' error generally resolves itself and is not uncommon if you send lots of emails in a day. If the problem persists, you might reinstall 91.5.

I have this problem as well, and my isp/email provider is TDS as well. It just started happening a few days ago, not associated with any updates of TB. In fact, it started happening at the same time in a few different versions of TB I have on various devices.

I changed no email settings anywhere, and TB has been working flawlessly with tds.net for many, many years. My theory is that TB does something slightly different with some part of the handshake, which TDS has let slide until now. Now they've tightened up compliance, and TB gets caught up once in a while.

Incidentally, I have tried a couple other email clients, and they have no problems. The TB/TDS problem is weird because it is intermittent. I was having problems with IMAP for a couple of days, too. In any case the 'too many messages' error is nonsensical - I send maybe 1 or 2 emails a day. The error is not associated with any email address i am sending to.

I believe there is some slop in the TB/TDS smpt handshake. I hope this gets straightened out.

Thanks for the replies. When it was happening to me, unfortunately, TDS was having email issues that day, and not even their technicians knew about it. I told one of their techs that one of the port numbers that they recommend would not work (had to use Thunderbirds port number). She put me on hold to talk to her supervisor, and she comes back and then let me know that they were having an email issue all day. This was at 7:00pm. I started this problem at 8:00am. So all of their first line techs did not know about this.

Tried "fixing" the problem myself by uninstalling and reinstalling TB a couple of times, deleting my TDS account also. The tech at 7:00pm was the third tech I talked to. The second tech helped me for 2 hours and could not get it to work, sent me here. The next day I get a robo-call saying that the issue was fixed. But I still have problems. If I keep sending, I get the 'too many messages' error, but it eventually goes if I keep clicking send. Sometimes I have to enter my password too. I may switch to a different provider. All of this while I am trying to buy a house. Argh!  :-)

When encountering sending issues, having a log of the connection is very handy as there are no questions about what occurred. I have notes on creating a send log in my blog here. https://thunderbirdtweaks.blogspot.com/2021/10/logging-mail-sending-in-thunderbird.html

The log will contain entire emails, where the email is transmitted, and not all emails are suitable for public consumption. When posting content you might want to be a little careful, or craft short specific emails when you are having issues to log is complete and contain nothing inherently "personal"