The same thing happened to me this morning. My email account is a POP3 account that I got free from GoDaddy years ago. They have since sold it to Microsoft and now I have to pay an annual fee for it. I got an email a few weeks ago that Microsoft would no longer accept Basic Authorization from email apps like Thunderbird, and if I didn't change it my email would be shut off on June 2, 2023. They also made it sound like you had to switch your account from POP3 to Exchange, but further reading led me to believe this was not true for Thunderbird, since Thunderbird can use OAuth2 authentication. So I switched both the incoming and outgoing servers to OAuth2. They both worked fine, and June 2 came and went with no problems. Now it is June 9 and I am getting the same message as you for my incoming server, but my outgoing server is still working just fine. I am holding off switching my account to Exchange because it has limitations I don't like, and POP3 is perfect for me. I am hoping someone comes up with a solution. Until then I am using Outlook on the web. Also my phone email, the Samsung Email app on my Galaxy android, also stopped syncing. So it's not just Thunderbird.