nslookup mail.mymailserver.com
and another nslookup on the IP you got back from the previous query, likenslookup 1.2.3.4
which should give you back mail.mymailserver.com.I created a block rule for Port 25 in my Cisco Router. This is what is being blocked.If he shut down port 25, it won't be detected as an open relay at the moment, but it can't stay like this for long if he wants to use it.
The MAC address is that of the router and the server.
Check /var/log/mail.log, see who submits the messages.
Tamas