I've encountered the same issue. Always assumed that the outgoing mail server was taking a temporary vacation.

Yesterday was the most recent occurrence that lasted around 5 minutes, then magically resolved itself. What was curious is that during that period I sent a couple of test messages to myself. If I left the msg body blank, msg went out w/o delay. If I included the text, with one http link, then I got the never ending "Connecting to _____" msg.

I'm using T-bird and my outgoing server is configured to use SMTP relaying services.

I too would be interested in hearing what Mozilla has to say about this issue.