MX records set the location of the server that all electronic mail will be sent to. There can be as many backup servers as you like, each one with its own entry, and each given a priority. The lower the number, the higher the priority. If one fails, it goes to the next in the list. MX records can only be pointed to a host, never an IP. This is why a mail.your-domain.com host needs to be set up to point to the IP of the mail server, then the MX record points to that host.
