This HOWTO configures a DNS server to provide a 'kinda' fail-over service when the primary mail service is off-line.
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, 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 at a separate geographic location, would typically configured as a simple relay (or forwarder) with a very long retry time in which case it will accept the mail and try and relay it to the proper destination (mail.mydomain.com) over the next six weeks or whatever you configure the retry time to be.
Beware: There are a number of articles around the web which suggest that most mail systems do a poor job of using the 'preference system of the MX record.
3 reverse map
4 dns types
5 install bind
8 dns records
12 bind api's
13 dns security
bits & bytes
notes & tips