kbkestle original problem "I have an email that has a 4MB attachment that will not send. Outlook 2003 keeps trying to send but is unable to" , I believe is a bug in MS Outlook.
It seems the problem occurs with the combination of large attachment, slow connection (dial-up) and busy server. Outlook (not the server) times out and then starts all over again. Quite often, the message will be completely sent, but Outlook does not get advised, so it sends again, resulting in the recipient getting the message several times.
I found that increasing the Outlook timer to 10 minutes greatly improved the situatiuon, but is not the solution.
Tools -> email accounts -> (select account) Change - > More settings -> Advanced -> move the slider to full.
Hope it helps you as well.
Cheers,
Brodie