From: Claus Assmann (ca+OpenBSD_misczardoc.endmail.org)
Date: Wed Mar 01 2006 - 20:49:44 CST

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On Wed, Mar 01, 2006, Chris wrote:

> I want to set up a backup mx server to field incoming mail when my
> primary mail server goes down. I understand how to do this from a DNS
> standpoint, but what I don't know is what should be in my
> sendmail.mc/sendmail.cf file for this.

> Is there anything special I need to do for this? Anyone know any good
> documentation?

You need to allow relaying to the main server, see cf/README.

PS: from the sendmail X README:

Note about Backup MX Servers

It is not a good idea to run a backup MX server B for a host A that
has stronger anti-spam measures; if mails are sent to A via B, then
B may accept them for delivery, but A may reject them and hence B
has to sent bounces, which, in case of spam, are most likely to
forged addresses, hence those bounces will only cause additional
problems. The opposite case (B has stronger anti-spam measures than
A) can cause the rejection of mail that A actually wanted to receive.
Hence B and A should have the same anti-spam measures; i.e., a
system that acts as backup MX server for another one should perform
the same anti-spam checks as the main MX server(s).

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