At 15:34 -0700 on 10/13/2011, SM wrote about Re: SMTP traffic control:
It would also be helpful to have an enhanced status code that tells
a client: "don't even try one of the other MXs, it will reject you
too." That can be rather helpful for greylisting and for systems
that share traffic control information.
This one may be doable as currently the SMTP client will retry with
the next available host on a temporary error. As it would be a
significant departure from the existing SMTP specifications, it
would be better to do it through a SMTP extension.
The support that would allow the MXs to sync with the main MX (so
when the main MX blows off a sender with a "try again no sooner than
time" so will the secondary MX) would also help to stop those
spammers who try to send to a secondary MX as a means of attempting
to bypass spam controls at the main MX. Any attempt to connect to the
secondary MX when the primary MX is available and accepting messages
would cause the secondary to blow off the sender.