Editing MX record (section)

==Overview==
Resource records are the basic information element of the Domain Name System (DNS). An MX record is one of these, and a domain may have one or more of these set up, as below:
{{sxhl|2=zone|
Domain			TTL   Class    Type  Priority      Host
example.com.	1936	IN		MX		10         onemail.example.com.
example.com.	1936	IN		MX		10         twomail.example.com.
}}

The characteristic payload information of an MX record<ref>In these examples, the domain name concerned is in the first column, the [[Time to live#DNS_records|TTL]] (time-to-live) in the second, and the third is the "record Class" (in this case IN for Internet) - then MX to identify the type of record. The TTL is a validity period, indicating when the information  must be refreshed from an [[authoritative name server]]. 
</ref> is a preference value (above labelled "Priority"), and the domain name of a mailserver ("Host" above). 

The priority field identifies which mailserver should be preferred - in this case the values are both 10, so mail would be expected to flow evenly to both ''onemail.example.com'' and ''twomail.example.com'' - a common configuration. The host name must map directly to one or more [[A record|address record]]s (A, or AAAA) in the DNS, and must not point to any [[CNAME record]]s.<ref>RFC 2181, Section 10.3, ''Clarifications to the DNS Specification'', R. Elz, R. Bush (July 1997)</ref>

When an e-mail message is sent through the Internet, the sending [[mail transfer agent]] (MTA) queries the Domain Name System for the MX records of each recipient's [[domain name]]. This query returns a list of [[host name]]s of mail exchange servers accepting incoming mail for that domain and their preferences. The sending agent then attempts to establish an SMTP connection, trying the host with the lowest "Priority" value first. The system allows [[high-availability cluster]]s of mail gateways to be built for one domain if necessary.<ref name=balance>[http://zytrax.com/books/dns/ch9/rr.html HOWTO - Configure Round Robin and Load Balancing], Page modified: February 28 2014., zytrax.com</ref>

The MX mechanism does not grant the ability to provide mail service on alternative [[port number]]s, 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.