[Dovecot] Dovecot security

Hi,

THIS IS URGENT

I have Debian Linux machine which I installed as a mail server with postfix, and dovecot. my mail server is setup to use SMTP relay. I currently have ports 143, 995, 25 & SSMTP ports open. in the last few days I have been under attack where email is being sent to fake email address for example xxx@evg-mail.org which does not exist in the mysql db.

I need to figure out and lock down dovecot, because I believe the attack is some kind of virus /spyware. I need to know what statement in dovecot.conf or main.cf (postfix) I can modify to lock it down. Also open to install software to combat this kind of attack. Let me know what configuration files, info do you need to help out

Many Thanks

~Jay