Exchange 2000 Security Question

Help! I have supposedly locked down my server from being an open Relay. (According to the tech documents at MS). However, I checked my log and every so often someone is still relaying through my system!

When messages are bounced back, this is the message they carry:

vtqi@yahoo.com.tw on Fri, 13 Sep 2002 11:47:41 -0700
There was a SMTP communication problem with the recipient’s email
server. Please contact your system administrator.
” (envelope sender is )>

There is no linda defined in my Exchange Users, and I am behind a firewall (obviously,port 25 is open).

Any ideas? Today I am getting nailed with a lot of relays! What do I do?