I think our mail server has been compromised. Today morning when I checked, there were 1298 refused mails from yahoo. with this message `Messages from x.x.x.x.x(our ip) temporarily deferred due to user complaints - 4.16.55.1; see http://postmaster.yahoo.com/421-ts01.html)`

Mails are getting delivered to gmail though. I am new to this, can anyone please suggest me where should I start looking ?
We are using Postfix and dovecot on Ubuntu Server 10.04. And I followed this guide here <https://help.ubuntu.com/community/MailServer>