I suggest you contact the email provider and request the email settings for incoming and outgoing server. Thunderbird maintains a database of commonly known email providers, but does not have all of them. Once you have the information, we can assist. thank you.

I'm the one who checked the "I have this problem, too" button. I have been fighting a similar problem in Thunderbird 150 / Windows 11 for weeks. I found when I started to move some accounts from Thunderbird 140 to 150, the format that the questioner cites would not work. An example is lastname1FI@oldserver.net residing on a new server such as hoo.com, with an existing, but not related, email of lastnameFI@hoo.com. This works in 140 under Ubuntu, but not on 150 under windows 11. The fact that it works on 140, says that it has nothing to do with the server, in my opinion. I haven't tried 150 on ubuntu, but am convinced that it would behave the same. I am guessing that 150 uses only hoo.com to index passwords and not the complete name@hoo.com, but I hope I am wrong. I hope you will forgive the length of this, but I wanted to lay out my entire experience.

That may be a different problem. The original poster did not indicate use of a new server. That's its own can of worms. If you are having a problem with this, I encourage opening a new thread and supply more details. Thank you.