An 'MX' record or mail exchange record maps a domain name to a list of mail exchange server for that domain.
eg:
mydomain.com. 14400 IN MX 0 mydomain.com.
The 'MX' record shows that all emails @mydomain.com should be routed to mailserver mydomain.com.
Using multiple 'MX' record syntax
If you want to use multiple mail servers you have to use MX record preferences.The MX record preference values indicate which mail server to use and in which order to try them when they fail or don't respond. A larger preference is less prefered. Thus a mail exchanger with a preference of zero(0) is always prefered over all other mail exchanger. Setting preference values to equal numbers make mail servers equally prefered,
eg :
mydomain.com. 14400 IN MX 0 mydomain.com.
mydomain.com. 14400 IN MX 30 server2.mydomain.com.
