HOWTO - Configure Mail Servers fail-over

This HOWTO configures a DNS server to provide a 'kinda' fail-over service when the primary mail service is off-line or too busy to accept mail. (MX RR details.)

Define MX records with different priorities e.g.

; zone file fragment
      IN  MX  10 mail.example.com.
      IN  MX  20 mail.example.net.
....
mail  IN  A      192.168.0.4

If the most preferred mail server (mail.example.com) is down or not resonding, mail should be sent to the alternate server (mail.example.net). The server mail.example.net, which in the fragment above is external to the domain and ideally is hosted at a separate geographic location, would typically configured as a simple relay (or forwarder) with a very long retry time (at least 2 - 7 days) in which case it will accept the mail and try and relay it to the proper destination (mail.example.com) over the next 2 - 7 days or whatever you configure the retry time to be.

Beware: There are a number of articles around the web which suggest that many mail systems do a poor job of using the 'preference' value of the MX record.

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