DNS on Windows Server 2003, 3rd Edition

MX Records

DNS uses a single type of resource record to implement enhanced mail routing, the MX record. Originally, the MX record’s function was split between two records, the MD (mail destination) and MF (mail forwarder) records. MD specified the final destination to which a message addressed to a given domain name should be delivered. MF specified a host that would forward mail on to the eventual destination, should that destination be unreachable.

Early experience with DNS on the Internet showed that separating the functions didn’t work very well. A mailer needed both the MD and MF records attached to a domain name (if both existed) to decide where to send mail—one or the other alone wouldn’t do. But an explicit lookup of one type or another (either MD or MF) would cause a name server to cache just that record type. So mailers either had to do two queries, one for MD and one for MF records, or they could no longer accept cached answers. This meant that the overhead of running mail was higher than that of running other services, which was eventually deemed unacceptable.

The two records were integrated into a single record type, MX, to solve this problem. Now a mailer just needed all the MX records for a particular domain name destination to make a mail-routing decision. Using cached MX records was fine, as long as the TTLs matched.

MX records specify a mail exchanger for a domain name: a host that will either process or forward mail for the domain name (through a firewall, ...