There's
just one more condition - his mate, though great as mates go, is an anti-
RBL purist. He refuses to use RBLs.
His mate is a wise man. RBLs are a really terrible idea, and they've
caused a lot of valid mail to be rejected. There's really no way to
reliably determine that a message is spam based on the IP address or
sender's domain name. The most you should do with RBLs is delay or
rate-limit mail from the blacklisted sites, you should never reject
such mail.
My proposal: an extension to the MX record in the DNS, which must be
backward compatible with existing MX records - that is, non-conformant
mailers must not be confused by the new form of the record.
Doesn't seem like a good idea in general, nor does it seem necessary.
It's perfectly reasonable for the secondary MX to be explicitly
configured to flush its mail queue for the primary MX at pre-determined
times (as in a cron job). It's also perfectly reasonable for the
primary MX to contact the secondary MX at regular intervals and
(using any of a variety of mechanisms) say "please give me my mail".
So it doesn't seem like we need a new DNS RR to solve this problem.
Keith