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>3.1. Multiple Incoming Mail Exchangers</H1
><P
> Most domains list more than one incoming <A
HREF="gloss.html#mx"
><I
CLASS="glossterm"
>Mail Exchanger</I
></A
>s
(a.k.a. <SPAN
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>"MX hosts"</SPAN
>). If you do so, then bear in
mind that in order to have any effect, any SMTP time filtering
you incorporate on the primary MX has to be incorporated on all
the others as well. Otherwise, the sending host would simply
sidestep filtering by retrying the mail delivery through your
backup server(s).
</P
><P
> If the backup server(s) are not under your control, ask
yourself whether you need multiple MXs in the first place. In
this situation, chances are that they serve only as
<EM
>redundant</EM
> mail servers, and that they in
turn forward the mail to your primary MX. If so, you probably
don't need them. If your host happens to be down for a little
while, that's OK -- well-behaved sender hosts will retry
deliveries for several days before giving up
<A
HREF="greylisting.html#FTN.noretrysenders"
><SPAN
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>[1]</SPAN
></A
>.
</P
><P
> A situation where you <EM
>may</EM
> need multiple
MXs is to perform load balancing between several servers -
i.e. if you receive so much mail that one machine alone could
not handle it. In this case, see if you could offload some
tasks (such as <A
HREF="datachecks.html#virusscanners"
>virus</A
> and
<A
HREF="datachecks.html#spamscanners"
>spam</A
> scanners) to other
machines, in order to reduce or eliminate this need.
</P
><P
> Again, if you do decide to keep using several MXs, your backup
servers need to be (at least) as restrictive as the primary
server, lest filtering in the primary MX is useless.
</P
><P
> See also the section on <A
HREF="greylisting.html"
>Greylisting</A
> for
additional concerns related to multiple MX hosts.
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