What are MX records?

An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. MX records point to the servers that should receive an e-mail, and their priority relative to each other.

An MX record must contain a host name defined by an A record.
An A record or address record maps a hostname to a 32-bit IPv4 address.

When an e-mail message is sent through the Internet, the sending Mail Transfer Agent (MTA) e.g. Microsoft Exchange Server, 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 of host names of mail exchange servers accepting incoming mail for that domain, together with a preference number. The sending agent then attempts to establish a Simple Mail Transfer Protocol (SMTP) connection to one of these servers, starting with the one with the smallest preference number, delivering the message to the first server with which a connection can be made. If no MX records were present, 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 and the order in which they should be tried, increasing the likelihood that mail may be delivered and providing the ability to distribute the processing of incoming mail across multiple physical servers. This ability to run multiple mail servers easily is proving very valuable for high-availability clusters of inexpensive mail gateways that can then process hundreds of messages per second in aggregate to quarantine or remove spam and/or viruses. However, not all versions of all mail transfer agents pay attention to lower priority MX records — in other words, if the highest-priority MX server fails, the MTA doesn’t address the backup server.

The MX mechanism does not grant the ability to provide mail service on alternative ports, nor does it provide the ability to distribute mail delivery across a set of equal-priority mail servers by assigning a weighting value to each one.

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