MX record - what is it?

An MX record, or mail exchanger record, — is a type of resource record in the domain name system that specifies a mail server responsible for accepting email messages on behalf of recipient's domain and a preference value used to prioritize mail delivery. Set the mail exchanger records of your domain name specifies how email should be routed using the Simple Mail Transfer Protocol (SMTP).

MX record: an overview

Resource Records are the main information element of the domain name system (DNS). They differ in the type identification (A, MX, NS) DNS class (Internet, CHAOS). Records have a validity period (lifetime) assigned to them, indicating when the information they store needs to be updated with the authoritative name server. Resource records are organized within the DNS based on their FQDN of the recipient's e-mail (part of the name after the @symbol).

The Characteristic payload information of an MX record is the FQDN for mail host and a preference value that should be displayed directly in one or more address entries.

When the email is sent via the Internet, the sending mail transfer agent (MTA) queries the domain name system for MX records for each recipient's domain. This query returns the list of hosts of mail exchange servers accepting incoming mail for that domain. The sending agent then attempts to establish an SMTP connection.

Basics of prioritization

In the simplest case the domain can have only one mail server. For example, if an MTA looks at the MX records for example.com and the DNS server responds only with mail.example.com with the number of preferences 50, the MTA will attempt to send mail to the specified server. In this case, the number 50 can be any integer permitted by the SMTP specification.

But when the query returns more than one MX server room preferences for each record determines the relative priority of a specified server. When a remote client (typically another mail server) searches the MX for a domain name, it receives a list of servers and their preference numbers. Should initially be verified by any server with the lowest number of preferences. To provide reliable mail transmission, the SMTP client must be able to check each of the relevant addresses in this list in order until the delivery attempt succeeds.

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Load balancing between arrays of the mail servers

The Method used to distribute the load of incoming mail on the server array should return the same preference number for each server in the set. When determining which server has equal preference to send mail, the sender must randomize them to spread the load across multiple mail exchangers for a specific organization. Multihomed servers are treated differently, as in this case any randomization is already used by name server. This applies mainly to routing problems. Other types of server load can be solved by using a proxy SMTP server.

Backup

A Target server, i.e. one that knows how to deliver to Inbox of the corresponding user is typically the most preferred. Servers with lower priority, the so-called backup or secondary MX record usually store the messages in the queue, waiting for the primary server. If both servers connected to the network or somehow connected to one another, the backup MX will forward the letter to the primary mail exchanger. Backup acts as a repository.

How to configure MX record: priority

Email is sent to the exchange server with the lowest preference number (highest priority), so the mail exchanger record, used for routing should have the lowest preference number, typically 0.

Priority determines the order, which should be linked to the servers (if multiple servers with different priorities). The first will be checked server with the highest priority and lowest number of preferences. In DNS records, as a rule, preference is selected and specified.

Error during setup

A Common misconception about the order of preference of the MX record of the domain is that it is designed to increase the deliverability of mail. However, the mere use of multiple records with the same preference provides this benefit.

Another common misinterpretation of the order of the MX preferences is that it is designed to provide “failover” in the case of server overload. Although it can be used in this way, it is a bad method of resource management, because it intentionally creates overload, not fully utilizing all available equipment and allows to check MX record. Assigning the same value to all available servers provides the same advantage can help avoid overload situations and thereby increase system throughput by reducing delays.