Mail server help
 

...

You are saying you are running an open relay?

If this server was running properly previously then you have been hacked, take the box offline immediately and re-install from scratch, making sure all security updates are installed and all security guidelines are followed.

.....

.....

If mydomain.com is local (in mydestination for postfix) it will always accept mail to that domain - that's how it works.

.......

So what little information you did post is bogus - good luck...