From: Simon Waters (simonwzynet.net)
Date: Wed Mar 01 2006 - 04:35:04 CST

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On Wednesday 01 Mar 2006 09:53, Brendan Grossman wrote:
>
> I've been told (not sure with postfix) that backup MXs can be configured to
> hold mail indefinitely or for a certain time period.

They can do whatever you want. However indefinitely would be a bad idea, as
people would never know if mail wasn't delivered.

> Is this possible with postfix?

Yes

> The only option I've seen to configure a
> backup MX is with 'relay_domains domain.com' set on the backup MX of
> domain.com - which, I assume, instead of the sender's mail server
> attempting to redeliver, the backup MX will.

Pretty much the idea.

> As a matter of fact I'm not quite clear on how backup MXs work. If the
> above is correct, and the primary MX doesn't come back in x number of
> hours, will mail be bounced?

Yes.

If you aren't clear how backup MX work, I'd strongly suggest not having one.
They made sense when Internet email was more gentlemanly, and connectivity
was a more hit and miss affair than it is today, and email was a substantial
part of ones bandwidth. When you would make sure your email got somewhere you
could reliably talk to, and ideally talk to cheaply.

Now it is more sensible for people to keep the configuration as simple as
possible, and let the senders server retry as long and as often as it likes
(or not).

The main use for extra MX records these days it to load balance across
multiple identical mail servers - and we don't have enough email to justify
that complexity here - ymmv.

> And if so, will the sender's mail server
> attempt to redeliver or just return it to the sender?

It will return it to sender.

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