I'm going to join Scott on that limb and suggest your problem is that the server won't relay e-mail messages from other computers. This is standard configuration for SMTP servers nowadays.
Probably those other systems can relay because they are authenticating themselves with the server. Your program may have to do the same. At any rate, the administrator of the SMTP server should be able to clear up any doubts you might have. Go and ask.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com