I've been doing some quick looking around but was unable to locate any 
sendmail rules regarding blocking an incoming message based on misconfigured 
DNS on the sender's side.

Is there a way to do this? Or am I going to have to set up my own dns 
blacklist and use something custom that checks the reverse and returns a 
false a la dnsbl or ordb?

I'm sure there's something simple. But I've long since forgotten how to 
actually write sendmail rules..  dang those m4 macros anyway! :)

Thanks!
-sudog