Multihoming single server in DNS

Hi, I am installing a mail server for a client who wants a fail-over mechanism, so if the primary IP address goes down then the secondary IP (a cheap ADSL with static address) can be used with the mail service automatically. I am purchasing a dual WAN router, How do I complete this in DNS? If I use the same MX priority it will create a round-robin and I don't want to shorten the TTL of the MX records, as it feels like an ugly way of accomplishing what I want. Normally I would do the following: server.domain.com - A record - IP Address domain.com - MX record - 10 - server.domain.com How do I have to change this to get a multi-homed single server functioning in the case of the primary line going down? I did have a suggestion that I CNAME the A records and use those hosts with the mx's, but I didn't know exactly what and how that would help... Thanks in advance, Spongy