I have had this problem for some time. I hit send and either get a "message was not sent" with no explanation or RC grinds away for 2 minutes and leaves me on the message page with the message not sent. It only happened when I replied and about once in 50 messages. My web host just upgraded to a new version of RC and now it happens every 4 messages and even if I am sending a brand new one. I have no problems using the same email with squirrelmail, dwmail, horde or other programs, just RC. My web host is stumped and suggested I try this forum. I tried reporting it in the bug tracker and got a message that this is not where to post bugs. So I'm trying here.
What in the error log?