Exchange SMTP sending emails FROM forged domains

[jcas...@gmail.com]

I've got an exchange 2003 server that is sending out emails from other
domains and I can't figure out where it's coming from. For example,
the server hosts mydomain.com, but if I look in the queues or smtp
logs I see tons of outbound emails with "mail from:
fr...@notourdomain.com".

The server passes open relay tests on a few sites I've used and I've
tested it manually to verify. On the SMTP virtual server I have
relaying set to only allow the following list, but the list is empty.
I have also unchecked "allow any computer that authenticates to
relay".

So can anyone help me figure out how this server is sending emails
from forged domains. Also, how can I identify where on the network
these emails are coming from. Thanks.