Please note I don't want to take anything away from roosrj's tip which displays infinitely more in depth knowledge about the SMTP protocol than I can - as a lazy man's alternative I've always used PostCast v2.6 to test anything mail related: http://www.postcastserver.com/download/release.aspx?p=3[^].
I just set it up to listen on a port and don't bother with forwarding the mail on. This way I can use it to not only test my mail sending code, but I also get to see the contents of the mail (useful for inspecting HTML mails), switch between HTML and RTF, and so on - as if I'd sent the mail to my Outlook.
