MX record

Domain Name System specifying how Internet e-mail should be routed.

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 (the portion of the e-mail address following the "@"). This query returns a list (possibly empty) 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 MX records exist for the recipient's domain, the sending agent makes a DNS query requesting an A record for that domain. If successful, it attempts to establish an SMTP connection to that address.)

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.