From: Aaron Wolfe (aawolfegmail.com)
Date: Sat Nov 03 2007 - 15:29:24 CDT

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On 11/3/07, Victor Duchovni <Victor.Duchovnimorganstanley.com> wrote:
> On Sat, Nov 03, 2007 at 04:09:12PM -0400, Aaron Wolfe wrote:
>
> > > If you run more than one MX, and you are interested in a consistent
> > > experience, consider using (on both MXs) a greylist package that supports
> > > a shared database (whether that DB is filesystem, BDB or SQL).
> >
> > You don't really need a shared database, simply using greylisting on
> > all of your incoming servers will give essentially the same results.
>
> No, the previous comment is right. Postfix, for example, will only
> try two SMTP sessions for a single delivery, so if one has more than
> 3 primary MX hosts, it is possible for a retry to hit two new hosts,
> that have not seen the triple (client, sender, recipient) before.
>
> With 4 MX hosts, the probability of a retransmission avoiding the original
> 2 MX hosts is 2C2/4C2 or ~17%. With 6 MX hosts, 4C2/6C2 or 40% and it
> is even possible for a second re-transmission to avoid all 4 of the
> previously tried hosts with a probability of ~7%.
>
> With 3 or fewer MX hosts, at least one of the two re-tried will have
> been tried during the previous delivery attempt.
>

uh.. yeah. but he has 2 servers and only one is primary MX.

-Aaron

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