From: Wietse Venema (wietseporcupine.org)
Date: Thu Nov 01 2007 - 10:51:20 CDT

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gordanbobich.net:
> Sure - and I've gone one better and hidden my real MX somewhere between
> the rejecting ones at the top (which leads to immediate retries to the
> next MX down, which may or may not do the same thing), and the tarpitting
> ones at the bottom. And even if a valid MTA gets to the bottom ones
> through a minor network outage, it'll still eventually time out and roll
> over to retry from the top after a little while.

The RFC does not require that an SMTP client tries every MX host.
It only requires two. If you list lots of non-responding MX hosts
then you can expect interoperability problems.

Postfix will not connect to all MX hosts, because that is a way
for bad guys to DOS a server.

        Wietse

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