Overview
Resource records are the basic information elements of the Domain Name System (DNS). They are distinguished by a type identification (A, MX, NS, etc.) and a DNS class (Internet, CHAOS, etc.). The records have a validity period (time-to-live) assigned to them, indicating when the information they hold must be refreshed from an authoritative name server. Resource records are organized within the DNS based on their name field, which is a fully qualified domain name (FQDN) of a node in the DNS tree. In the case of an MX record, this specifies the domain name of a mail recipient's email address, i.e. the portion after the @ symbol that delimits the recipient's account name.
The characteristic payload information of an MX record is the fully qualified domain name of a mail host and a preference value. The host name must map directly to one or more address record (A, or AAAA) in the DNS, and must not point to any CNAME records.
When an e-mail message is sent through the Internet, the sending mail transfer agent queries the Domain Name System for MX records of each recipient's domain name. This query returns a list of host names of mail exchange servers accepting incoming mail for that domain and their preferences. The sending agent then attempts to establish an SMTP same domain.
The MX mechanism provides the ability to run multiple mail servers for a single domain, and allows administrators to specify an order in which they should be tried. This ability to run multiple mail servers proves very valuable for high-availability clusters of inexpensive mail gateways, which can then process hundreds of messages per second in aggregate to quarantine or remove spam and/or viruses.
The MX mechanism does not grant the ability to provide mail service on alternative port numbers, nor does it provide the ability to distribute mail delivery across a set of unequal-priority mail servers by assigning a weighting value to each one. MX can be used to distribute delivery across equal-priority mail servers.
Read more about this topic: MX Record