I was able to check with my service provider. They were able to trace some spam mails passing through my IP, that is why they blocked the SMTP service.
Now my problem is how do I check and block these emails passing through my server. I have already tested the server using rkhunter and chkrootkit, but there were no significant alerts that would say I have open relay.
I hate to do a fresh install/configuration of the mail server.