Not quite mail relay

[Derrick Ryalls]

I am looking for a way to further secure a mail server.  It isn't an open
rely, but when others try to use it as such with bad return addresses, a
small flood of rejection mail end up on the bad addressed server.
 
Ex.
 
To: blah at msn.com
From: fake at hotmail.com
 
hotmail ends up with a ton of bounce msgs and thinks the server is a relay.
How would I go about just dropping those msgs completely?
 
Qmail is the mail server, but I was hoping someone would have an idea.
 
-Derrick