Short form for mail exchange record, an entry in a domain name database that identifies the mail server that is responsible for handling e-mails for that domain name. Mail exchange" is just another name for the machine whose primary function is receiving and sending email.

When more than one MX record is entered for any single domain name that is using more than one mail server, the MX record can be prioritized with a preference number that indicates the order in which the mail servers should be used. The preference values of a host's mail exchangers determine the order in which a mailer should use them. Mailers attempt to deliver to the mail exchanger with the lowest preference value (highest priority). The most preferred mail exchanger has the lowest preference value. If the highest priority mail exchanger fails, the mailer will attempt to deliver to the next highest priority.

 

The mail exchanger either delivers it to the individual host it is addressed to or gateways it to another mail transport.