One possibility is to have postscreen listen on multiple IP addresses with different MX preferences, and to require that clients don't connect to the secondary port before the primary.
The problem I see with this is that a site with multiple MX hosts
would need to share the postscreen database. I'm concerned that a
networked SQL database would introduce unacceptable latencies and
that it the postscreen database would become a single point of
failure for multuple MX hosts.
Wietse
