Stands for mail exchange record, a resource record in Domain Name System (DNS) databases that is used to implement mail routing.
MX records specify the host that will be used to process or forward Simple Mail Transfer Protocol (SMTP) mail for a given DNS domain. MX records are required for SMTP hosts to enable SMTP mail to be exchanged over the Internet.
An MX record specifying that incoming mail directed toward the microsoft.com domain should be delivered to the SMTP host mail.microsoft.com might be
microsoft.com. IN MX 0 mail.microsoft.com.
The number 0 is the preference value that specifies the priority of the SMTP host (which is used if more than one SMTP mail exchanger exists for a given domain).