I ran across a website that is dedicated to SMTP with SPF filtering
(sender policy framework).  After reading the write-up, it seemed like a
good idea and it made actually made sense but like many things, there is
a difference between sounding feasible on paper (or even e-paper) and
the actual implementation of it.

Has anyone used SFP?  If you, have what is your opinion of it? Was it
easy to setup? 

http://spf.pobox.com/howworks.html

Thanks,

Michienne