Not sure about the code  as I'm code dyslexic, this looks a relay issue and
not a code issue.
you may need to go into Internet Information services and set  the virtual
SMTP access properties, there is a relay restrictions section that you can
set.
Your network may not allow relaying at all?

I have a Web Objects system  that allows you to set a username and password
for a mail  account so it doesn't use relaying, I'd be interested to know if
and how you could code that in aspx. so it wouldn't use relaying?