SPF. reverse lookups. It's the "How many half-baked ideas can we fit into a single thread?" contest, again.

[Jonathan de Boyne Pollard]

L> What am I missing?

You are missing years of discussion where it has been pointed out, time 
and again, that reverse lookups aren't a security or a validation 
mechanism, that reverse lookup relies upon attacker-supplied data, that 
reverse lookups are not the inverses of forward lookups, and that 
SMTP-based Internet mail is a store-and-forward system where any 
particular SMTP Relay client is not necessarily the actual origin of a 
message.

<URL:http://homepages.tesco.net./~J.deBoynePollard/FGA/dns-avoid-double-reverse.html>
<URL:http://homepages.tesco.net./~J.deBoynePollard/FGA/smtp-spf-is-harmful.html>

SPF and reverse lookup validation have been discussed in a lot of fora 
many times over a long time, and eventually most reasonable and 
intelligent people reach the same conclusions.  Google is your friend.  
Search and read.