"Postmaster" is a required email address for a mail server to serve Non Delivery Reports (NDRs) to let someone sending mail in know that the email didn't arrive due to some reason.
Spammers take advantage of this by sending large volumes of emails to "stupidname@domain.com" and then redirect the "bounce" back to "someoneelse@anotherdomain.com".
This is what is known as "backscatter".
Unfortunately unless you have control of the actual mail server itself there is no way you can control backscatter.
There are 2 fairly simple fixes for this.
1 - write rule into your mail client saying that anything coming from postmaster@ get deleted or
2 - use a decent anti spam/antivirus program.
Note: if you use method 1 then if you accidentally make a typo sending an email to someone (like steak@ozstoc.com), it will not get there and you will never know.
Hope this helps.
Postmaster, is your email system.
You may be confused with postfix ....
