An MX Record stands for Mail Exchange Record. This is the DNS record that specifies the mail server addreesses used to receive incoming mail for a specific domain. Typically, at least two mail server addresses are provided for a primary and alternate means of transmitting an e-mail to an address "@" your domain.
Since usually more than one mail server address is configured for use with a mail system, it is necessary to set priorites when entering the addresses. The priority specifies which address should be tried first to handle mail sent to that domain. If the primary mail server address would become inaccessible or malfunction, then the second mail server address with a lower priority would then be tried.
Mail server priorities are commonly set using multiples of 10, with the lowest number taking priority (much like golf). View the sample MX Records below:
Using the example above, for any mail sent to someone @domain.com, the MX records state that the mail should be routed to mx00.domain.com, since its priority is the highest (has the lowest number). If the mail server at mx00.domain.com cannot accept mail at this time, then the mail can be routed to mx01.domain.com as an alternate means of transmission.