[exim] testing for non existent domain in From: header?

A colleague runs sendmail and rejects messages if their From: header
uses a non existing domain. He's suggested to my boss that we do the
same (helpful chap) as an anti-spam measure.

I can't find anything in the list archives nor do I remember it coming
up in the past. Can anyone either (i) point me to an implementation of
this or (ii) explain why it isn't a good idea?