limiting SPAM

Is there something which prevents spammers from sending their messages 
with a null reverse path in the envelope?  In other words, with
MAIL FROM: <>
in the SMTP exchange?

As I am developing a MTA in which I hope to limit spam by filtering on 
the reverse path, it looks to me like this opening intended for error 
messages might be a big hole in my security.
Thank you,
Rich Hammer
Hillsborough, N.C.
mailscreen.net