I am having the same problem, of Outlook 2003 not sending messages I compose. I select "send" on the message itself and it goes to my Outbox. I did note that unlike what I have seen before, messages that have not been sent are in italics, but the ones now are not. I select "send/receive" and get the dialogue box that shows sending and receiving, but the message remains in the outbox, and it not sent.
I have no error messages, no limits by ISP, no connectivity issues, as I do receive email that was sent by other people.