Definition of MX Record in the Network Encyclopedia.
What is MX Record?
MX Record stands for mail exchange record, is a resource record in Domain Name System (DNS) databases that is used to implement mail routing.
MX records specify the host that will be used to process or forward Simple Mail Transfer Protocol (SMTP) mail for a given DNS domain. MX records are required for SMTP hosts to enable SMTP mail to be exchanged over the Internet.
MX Record example
An MX record specifying that incoming mail directed toward the microsoft.com domain should be delivered to the SMTP host mail.microsoft.com might be:
microsoft.com. IN MX 0 mail.microsoft.com.
The number 0 is the preference value that specifies the priority of the SMTP host (which is used if more than one SMTP mail exchanger exists for a given domain).
The lowest-numbered records are the most preferred. This phrasing can be confusing, and so the preference number is sometimes referred to as the distance: smaller distances are more preferable.
Load distribution
The standard approach to distribute the load of incoming mail over an array of servers is to return the same preference number for each server in the set. When determining which server of equal preference to send mail to, “the sender-SMTP MUST randomize them to spread the load across multiple mail exchangers for a specific organization”, unless there is a clear reason to favor one.
An alternative approach is to use multihomed servers, where the one host returns several IP addresses. This method places the burden on the DNS system rather than the SMTP-sender to perform the load balancing, which in this case will present a list of IP addresses in a specific order to the clients querying the A record of the mail exchanger. Since the RFC requires that the SMTP-sender use the order given in the A record query, the DNS server is free to carefully manipulate its balancing based on any method, including round robin DNS, mail server load, or some undisclosed priority scheme.
External References:
- RFC 5321 – Simple Mail Transfer Protocol, October 2008
- RFC 7505 – A “Null MX” No Service Resource Record for Domains That Accept No Mail, June 2015
