Hey Paulson, I just had the same problem setting up my mail server. I could receive, but not send. Do you have a mail server that you use besides the web mail? (A "main" program?) How does that program integrate with IMail? In imail, do you need to set up the pop, smtp, dns servers?
My problem was with the DNS servers. I had signed up with Domain Direct, and asked them to change my MX record. They changed it, and set-up their DNS to point back to my server. So when I used their DNS to send mail, it pointed back at my computer and hopped back and forth between the two. I needed to change my DNS servers to get it to send correctly. Now works great.
What do you have your DNS set to right now? What other settings do you have? What are the hosts set to?