Thanks,  I hadn't thought about the backscatter problem.

If there is a proxy involved, then they HAVE to set (in amavisd) all
final destinations as 'DISCARD' and not BOUNCE.

I also think I will try to look at adding it to trusted networks in SA,
but excluding it from the internal networks in amavisd.

IP addresses are not the basis of p0f, so that doesn't work, and without
hacking SA SPF plugins, I can't see any way to trust SPF.

For the purposes of this discussion, the biggest reason I can't be on
the edge where Id like to be is that there is a massive proxy/load
balancer/failover device that does more than email.

Many firewalls 'proxy' the email also, so its not like you can take it
out.
(oh, turn off Cisco's smtp mail fixup was another thing I found when
dealing with odd ball setups)

Thanks to everyone who had real answers.
-- 
Michael Scheidell, CTO
Office: 561-999-5000 x 1259
Direct: 561-939-7259
Real time security alerts: http://www.secnap.com/news
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