MX record

In the simplest case, a domain may have just one mail server. For example, if an MTA looks up the MX records for example.com, and the DNS server replied with only mail.example.com with a preference number of 50, then the MTA will attempt delivery of the mail to the server listed. In this case, the number 50 could have been any integer permitted by the SMTP specification.

When more than one server is returned for an MX query, the server with the smallest preference number must be tried first. If there is more than one MX record with the same preference number, all of those must be tried before moving on to lower-priority entries. An SMTP client must be able to try (and retry) each of the relevant addresses in the list in order, until a delivery attempt succeeds.[4]

The standard approach to distributing a 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.[4]

An alternative approach is to use multihomed servers, where the one host returns several IP addresses.[3] This method places the burden on the DNS 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.

Some domains will have several MX records, one of which is intended as a "backup" - with a higher preference number so that it would not normally be picked as the target for email delivery.

However, in the case of errors from the lower-numbered hosts, (perhaps due to an outage of some sort), sending email servers will deliver to the "backup" host - queue.example.com in the example below:

Domain          TTL   Class    Type  Priority   Host
example.com.    1936    IN      MX   10         onemail.example.com.
example.com.    1936    IN      MX   10         twomail.example.com.
example.com.    1936    IN      MX   100        queue.example.com.

If the backup server has direct access to user mailboxes, mail will proceed there, but otherwise will likely be queued on queue.example.com until the outage is resolved.

In the absence of this sort of arrangement, when a domain's mail servers are all offline, sending servers are required to queue messages destined for that domain to retry later. However, these sending servers have no way of being notified that a previously offline domain's servers are now available, and so resort to a polling schedule - and will only discover that the domain is available whenever they next attempt delivery. The delay between when a receiving domain's servers come online and when delayed messages are finally delivered can be therefore anywhere from minutes to days, depending on the retry schedule of the sending servers - and the receiving domain has no visibility or control over this.

Spammers may deliberately direct mail to one of the backup (high distance) MX servers of a domain first, on the assumption that such a server will have less effective anti-spam filters. An anti-spam technique called nolisting is based on assuming this behaviour.

RFC 821 was published in 1982. It makes only passing references to DNS, because at the time the transition from HOSTS.TXT to the DNS had not yet started. RFC 883, the first description of the DNS, was published over a year later in late 1983. It described the experimental and little used MD and MF records. According to RFC 897 and RFC 921, the transition to DNS started in 1983, but HOSTS.TXT was not scheduled to be phased out until the end of 1985 and was not totally phased out until the late 1990s.

In January 1986, RFC 973 and RFC 974 deprecated the MD and MF records, replaced them with MX, and defined the MX lookup with fallback to A. RFC 974 recommends that clients do a WKS lookup[7] on each MX host to see if it actually supports SMTP and discard the MX entry if not. However, RFC 1123 changed this to say that WKS should not be checked.

This means that SMTP had been in use for at least a year using HOSTS.TXT, and then another couple of years using A, MD, and MF, before MX came along. MD and MF were hard to use, so most people just used the A record. Under the circumstances, MX without fallback to A would not have worked because of the substantial installed base of mail servers using A records. The early use of MX was to identify gateways to other networks, but it did not come into wide use until the DNS was well established in the early 1990s.[8]