Recently, I have received a lot of Mailer-Daemon type messages with undeliverable messages from mailservers I KNOW my users haven't been mailing to. Some are anti-spam messages from these mailservers spam-filters.
I suspect that someone is using my mailserver to relay spam. Also my mail.warn file contains many warnings like:
Apr 20 13:59:21 mail2 postfix/smtpd[16680]: warning: hostname verification failed: Name or service not known
These are all unknown hosts to me. So, as you see, I supect foul play.

How do I go about detecting what is going on, and how can I prevent my mailserver from being exploited?

Appeciate any help on this...

Regards Arnljot