Blocking mail supposedly from my domain

[Dennis Carr]

Over the past couple days I'm noticing mail coming in from outside that is 
supposedly from users of mine - but apparently isn't.  HELO message comes 
from chez-vrolet.net which is in my $mynetworks setting, but the IP 
address for the incoming machine does not match DNS.

What adjustment in main.cf should I look at?  On the surface, 
permit_mynetworks in strategic locations can be eliminated, but last time 
I did that, I couldn't send mail from localhost.

-Dennis