Re: [9fans] spam protection vs. secondary mx'es

[William Josephson <jkw@eecs.harvard.edu>]

On Wed, Nov 29, 2006 at 09:35:19AM +0000, Robert Raschke wrote:
> how do people use greylisting and /mail/lib/sender protection in the
> presence of a secondary MX?

For Unix hosts, consider mail avenger.  On secondary MXes,
mail avenger will contact the primary and reject/defer mail
when the primary MX is up.  Seems to work well in practice
since it is rarely the case that there is a route from the
secondary to the primary and the sender to the secondary but
not the sender to the primary and in that case mail can just
be deferred.  One can also greylist on the secondary as well
as the primary, but most senders (even well behaved ones) will
try the secondary as soon as the primary defers so you end up
with twice the greylisting delay.  I've also found that greylisting
is generally less effective than it was a year ago and that
SMTP call back is not really all that reliable.