I’ve had occasional issues with Thunderbird 3, sending out messages via smtp.gmail.com:465. I’m doubting it is my ISP at work, because I’ve been roaming around, and it seems to be the same issue. Long story short, if I click “Send Later” and batch the messages, it seems to be better than just writing a message, and sending it immediately – that seems to just bring up a message saying its connecting, and nothing happens for a long time, before I either cancel it or it times out.
Found some useful Mozilla documentation, on debugging Thunderbird using NSPR logging. Simply, you can do:
NSPR_LOG_MODULE also accepts POP3, and IMAP as arguments. The third line obviously only works on Mac OS X – on Linux, just replace with the path to thunderbird-bin.