from your main DNS provider you have to have an A record and an MX record for the secondary IP
IE
A records
mail.yourcompany.com 24.24.24.24 (primary IP)
mail2.yourcompany.com 24.24.24.25 (backup IP)
MX
mail.yourcompany.com priority 10
mail2.yourcompany.com priority 20
that should be a good way to setup your backup for your mail.