Some people have an intermittent problem when Thunderbird tries to save a copy of Sent mail back onto their mail server. The Thunderbird connection hangs after sending the message. The message is sent, and the message is uploaded to the server, but Thunderbird cannot send any more mail until it is quit and restarted.
The root cause of the problem is a bad interaction between Thunderbird and MIT's IMAP servers. There is no current solution. There are two workarounds: you can save sent mail into a local folder (not a server folder), or you can Bcc: all your mail to yourself, to get copies.