It sounds like the Windows 11 update changed how Thunderbird is handling outgoing mail. Since sending to Gmail works but other accounts fail, the problem is usually with the SMTP settings or how Windows handles secure connections after updates. A few things you can try to Double-check SMTP settings in your account settings. Specifically check outgoing settings. For sending emails via Gmail's SMTP server, the correct settings are: smtp.gmail.com as the server address, and either port 465 with SSL - OR - port 587 with TLS. You will also need your full Gmail address as the username and either your regular Gmail password