MX record

When an e-mail message is sent through the Internet, the sending mail transfer agent makes a DNS query requesting the MX record for the recipient's domain name, which is the portion of the e-mail address following the "@". This query returns a list of host names of mail exchange servers accepting incoming mail for that domain, together with a distance for each. The sending agent then attempts to establish an SMTP connection to one of these servers, starting with the one with the shortest distance, delivering the message to the first server with which a connection can be made. If no names were returned, a second request is made for the A record of the domain instead.

The MX mechanism provides the ability to run multiple mail servers for a single domain, both increasing the likelihood that mail may be delivered, and providing the ability to distribute the processing of incoming mail across multiple physical servers.

The MX mechanism does not grant the ability to provide mail service on alternative portss, nor the ability to bias the load distribution across a set of equal-distance mail servers by assigning a weighting value to each one. As of 2004, some MTAss, most notably exim, now support the use of SRV records for publishing the IP addresses, ports, distances, and weights of mail servers.

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