I have the same problem--but it only occurs randomly. Ii t occurs with all of my email accounts on my phone--home and work. I have found that when I discover that emails haven't been sent, if I restart the my Droid X, within a few seconds after the restart, the emails will send. I've gotten to the point that either early in the day, or on occasion later in the day, I'll send a test email to one of my other accounts so I know it's working. Not the way things should be. By the way--I do all my emails on 3G.
Strange ... What email APP are you using RobW ? I have YAHOO EMAIL. After composing and clicking SEND, my email goes out immediately. You don't have "SYNC OVER WiFi Only" set do you?