grep filters to block open relay

[hermit921]

My company put in an Exchange server, which got listed as an open 
relay.  They can't or won't configure Exchange to accept mail only if the 
recipient address is valid, and they won't put MailScanner in front of 
Exchange to do that.  Please accept this, don't comment, move on.

To block open relay functionality, they put pattern matching into place, so 
destination email addresses such as the ones used by 
relay-test.mail-abuse.org are rejected.  I had never heard of this approach 
before.  It blocks some legitimate email, of course.  Is this a common 
practice, or even a rare practice?  Any background information is appreciated.

hermit921