Redhat 9 - mail flooding problem

Jeg har en redhat 9 server stående, der hoster mail for 3 domænenavne.

Problemet er at jeg som administrator bliver oversvømmet af mails med  titlen

"Undelivered Mail Returned to Sender"

og indholdet:

This is the Postfix program at host nefetite.taxboel.dk.
I'm sorry to have to inform you that the message returned
below could not be delivered to one or more destinations.
For further assistance, please send mail to <postmaster>
If you do so, please include this problem report. You can
delete your own text from the message returned below.
The Postfix program
<hub@realize.dk>: unknown user: "hub"

Desuden er der vedhæftet en fil med indholdet:

This is a multi-part message in MIME format.
Dear user of realize.dk,
Your email account has been used to send a huge amount of unsolicited email during the last week.
Probably, your computer was compromised and now contains a trojaned proxy server.
Please follow the instructions in order to keep your computer safe.
Have a nice day,
realize.dk support team.

Hvad kan jeg gøre for at undgå flere af disse mails, og hvor hulen kommer de fra. Jeg har ikke sendt dem, og jeg kan ikke finde nogle  processer, der skulle sende dem på maskinen.


Jeg overvejer at sætte spamassasin op, men jeg har ikke lige luret hvorledes det kan gøres på den installation jeg har.