Secondary MX: Setup

Introduction

When there's only one mail server handling emails for a domain, if it becomes unavailable, emails sent by third parties to the primary server will be stored in the spool (outgoing queue) of the remote server for a few days in the best case, or immediately returned with an error message (depending on the problem or the configuration of the remote server).

Since problems and unavailability are inherent in modern computing, it's necessary to set up a system that can, at least in a degraded mode, transparently recover emails for the sender.

What does the Secondary MX do?

Its job isn't very exciting (well, I think that's the case for many web services)... It spends its time waiting for emails that arrive when the sending server couldn't deliver to or contact the primary MX. When it receives them, it keeps them in its spool and tries at regular intervals to contact the primary server to transmit these emails.

Don't forget to set up your mail server as an MX in DNS!

Configuration of Postfix

On the secondary MX

Main.cf

Edit the /etc/postfix/main.cf file and adapt (use only one of these):

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mydestination = burnin.deimos.fr, deimos.fr, burnin, localhost
relay_domains = $mydestination, mavro.fr

The form enclosed in [] eliminates DNS MX lookups.

By default, the SMTP client performs DNS queries even if you specify a relay machine. If your machine doesn't have access to the DNS server, disable DNS lookups of the SMTP client as follows:

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disable_dns_lookups = yes

For smtpd_recipient_restrictions, check that you have these two lines:

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smtpd_recipient_restrictions = permit_mx_backup, permit_mynetworks, reject_unauth_destination

Add this line to indicate the primary server:

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transport_maps = hash:/etc/postfix/transport

SMTP Banner: If you've set one with the machine name first, don't forget to change it to your secondary mx name:

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smtpd_banner = burnin.deimos.fr - Microsoft Exchange (5.5)

Then insert this line to indicate the people to relay:

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relay_recipient_maps = hash:/etc/postfix/relay_recipients

transport

Create a file /etc/postfix/transport and insert this:

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domain_to_relay      smtp:[primary_server_FQDN]

Example:

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deimos.fr     smtp:[fire.deimos.fr]
mavro.fr        smtp:[fire.deimos.fr]

relay_recipients

And now create the relay_recipients file:

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xxx@mycompany.com     x
xxx@mycompany.com         x

This must contain the names of the people to relay.

mailname

Edit the /etc/mailname file and put your DNS:

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deimos.fr

Validation

Let's validate everything now:

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postmap /etc/postfix/transport
postmap /etc/postfix/relay_recipients

And we reload the Postfix configuration:

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/etc/init.d/postfix reload

On the primary MX

Nothing to do :-)

Verifications

To see what we have in the spool:

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mailq

If you're in a hurry to retrieve your emails after bringing your primary server back up:

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mailq -q

References

Other documentation