Hi,

I have configured postfix email in my domain. Recent days I see lot of unknown emails going through my domain. ex: unknown-user@mydomain.com. I need you guys to show me a way to stop this. More than 100 emails are being sent from my domain. As a result I stopped my mail service now. Let me know ASAP.

Don't ask for "ASAP" help..we VOLUNTEER our time here, so expecting us to hurry and answer you is fairly rude. Especially since you've (essentially) asked this question before:
https://www.linuxquestions.org/quest...needed-942773/

Did you read/understand the answers? There are two links in there, related to securing a postfix server. What have you done/tried??? What have you researched on your own? What error(s) are you getting?? Environment you're in? Firewall(s)? ANY hints? I suggested in your other thread (since you said you were new to Linux), that you hire a consultant, since securing an email server/environment is a daunting task.