MX record

An MX record or Mail exchange record is a category of data in the 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, 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 ports, 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 MTAs, most notably exim, now support the use of SRV records for publishing the IP addresses, ports, distances, and weights of mail servers.

A primary point of confusion is how the priority system works for MX selection. The weighting or priority of an MX server is determined by the numerical value assigned in the DNS MX record. When a remote client (typically another mail server) does an MX lookup for the domain name, it gets a list of servers and their priority (or, as Daniel J. Bernstein notes, distance) relating to the domain being queried. The domain with the smallest numerical value has the highest priority and is the first server to be tried. The remote client will go down the list of servers until it successfully delivers the message or gets permanently rejected.

A favourite technique of spammers is to connect to the lowest priority MXs for a domain (those with the largest numerical value) in an attempt to avoid any anti-spam filters that may be running on the primary (highest priority) MX. Computer viruses have also been known to employ this technique in an effort to avoid anti-viral software.

External links

• "Moving a message closer to a remote recipient", part of Daniel J. Bernstein's guide to Internet mail infrastructure
• MX Record Lookup
• Domain Mail Server/Exchanger (MX Records) Lookup