---EDIT

http://support.microsoft.com/?id=828770


SEEMS like in 2003 server (and in some cases 2000) you can be an open mail relay even if you are not.

i disabled open mail relay on a few servers recently only to find they where still be used to send SPam, ive done a little digging and if you have this option turned on there is a way 4 spammers to send spam.
(please read above case id)