Hello there. Matt.welsh. We have read your message. We try to help you. Yes it can. https://support.mozilla.org/en-US/kb/cannot-send-messages https://en.wikipedia.org/wiki/Safe_mode http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-safe-mode http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-7 http://kb.mozillazine.org/Thunderbird_:_Issues_:_Cannot_Send_Mail -having time out errors. it is that you should just switch off your add -ons and watch with and without switching on again the same for your antivirus programs. Install or boot in safe mode for both Thunderbird and also for Firefox if that is your option. carefully reinstall your software for Thunderbird. And see if it is. (Mail back then). I have send you a lot of webpage information. With an upgrade or update, errors can sometimes occur because data can come together in a way. 421 Service not available. The server is shut down. 432 Problem loggin in. 450 Mailbox unavailable 451 Local error in processing 454 Temporary failure logging in 452 Insufficient system storage 500 Syntax error, or command unrecognized 501 Syntax error in parameters or arguments 502 Command not implemented 503 Bad sequence of commands 504 Command parameter not implemented 530 You must log in to use the server 534 Security problem when logging in 538 Security problem when logging in 550 Access denied 551 You must log in to use the server 552 Exceeded storage allocation 553 Mailbox name not allowed 554 Service failed, or not available. https://www.register.it/support//mozilla_thunderbird_imap.html?chglng=eng# You can try to register again , because of the new installation or upgrade! https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues Use a good antivirus Greetings Firefox Volunteer.