A mail exchanger record (MX 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 a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available. The set of MX records of a domain name specifies how email should be routed with the Simple Mail Transfer Protocol.
Overview
Resource records are the basic information elements of the Domain Name System (DNS). They are distinguished by a type identification (A, MX, NS, etc.) and a DNS class (Internet, CHAOS, etc.). The records have a validity period (time-to-live) assigned to them, indicating when the information they hold must be refreshed from an authoritative name server. Resource records are organized within the DNS based on their name field, which is a fully qualified domain name (FQDN) of a node in the DNS tree. In the case of an MX record, this specifies the domain name of a mail recipient's email address, i.e. the portion after the @ symbol that delimits the recipient's account name.
The characteristic payload information of an MX record is the fully qualified domain name of a mail host and a preference value. The host name must map directly to one or more address record (A, or AAAA) in the DNS, and must not point to any CNAME records.
When an e-mail message is sent through the Internet, the sending mail transfer agent queries the Domain Name System for MX records of each recipient's domain name. This query returns a list of host names of mail exchange servers accepting incoming mail for that domain and their preferences. The sending agent then attempts to establish an SMTP connection to one of these servers, starting with the one with the smallest preference number, delivering the message to the first server with which a connection can be made. If no MX records were present, the sending agent falls back to A, that is to say, it makes a request for the A record of the same domain.
The MX mechanism provides the ability to run multiple mail servers for a single domain, and allows administrators to specify an order in which they should be tried. This ability to run multiple mail servers proves very valuable for high-availability clusters of inexpensive mail gateways, which can then process hundreds of messages per second in aggregate to quarantine or remove spam and/or viruses.
The MX mechanism does not grant the ability to provide mail service on alternative port numbers, 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. MX can be used to distribute delivery across equal-priority mail servers.
MX preference, distance, and priority
According to RFC 5321, 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. An older RFC, RFC 974, indicates that when the preference numbers are the same for two servers, they have the same priority, hence those two terms are used interchangeably.
The basics
The relative priority of an MX server is determined by the preference number present in the DNS MX record of the recipient's domain. When a remote client (typically another mail server) does an MX lookup for the domain name, it gets a list of servers and their preference numbers. The smallest preference number has the highest priority and any server with the smallest preference number must be tried first. To provide reliable mail transmission, the SMTP client must be able to try (and retry) each of the relevant addresses in this list in order, until a delivery attempt succeeds. 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.
Load distribution among an array of mail servers
One technique used 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. Note that multihomed servers are treated differently, since in this case any randomization is assumed to have been applied already by the name server. This technique mainly addresses routing problems; other types of server load can be addressed by using an SMTP proxy.
The other alternative mentioned in the RFC is to use what appears to be a multihomed A record for a mail server. It may in fact be an array of mail servers that share the same host name. 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.
The backup MX
A target server, i.e. one that knows how to deliver to the relevant user's e-mail mailbox is typically one which is the most preferred. Lower priority servers, a.k.a. backup MX or secondary MX, usually keep the messages in a queue waiting for the primary server to become available. If both servers are online or in some way connected to one another, the backup MX will typically queue a message briefly and immediately forward it to the primary MX. The backup MX acts as a store-and-forward mail server.
Why have priority?
A common misconception about the MX preference ordering is that it is intended to increase the likelihood that mail may be delivered; however, merely having multiple MX records with the same preference provides this benefit (see below). Because the MX preference ordering specifies that some servers should be tried first, it is, if anything, a means of establishing load imbalance. Another common misinterpretation of MX preference ordering is that it is intended to provide a means of "failover" in the case of server overload. While it can be used that way, it is a poor resource management technique because it intentionally creates overload and does not fully utilize the available hardware. Assigning the same preference value to all of the available servers provides the same benefit and may even help avoid overload situations and thereby increase system throughput by decreasing latency.
The SMTP protocol establishes a store-and-forward network, and if 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. The sending servers will only discover that the domain is available whenever delivery of the delayed messages is next attempted. The delay between when a domain's servers come online and when delayed messages are finally delivered can be anywhere from minutes to days, depending on the retry schedule of the sending servers. This is the problem that backup MX records are uniquely qualified to solve. The idea is that the servers listed as secondary MX servers have some out-of-band way of knowing when the primary servers are back online. Thus, they are a more useful place to queue messages when the primary servers are offline than the original sender's queue.
The question then becomes: if the secondary servers are still online, why not give them the same priority as the rest of the domain's MX records? Secondary servers are ones that, for whatever reason, cannot or should not be used normally, but that can be used if the primary servers are offline. Reasons for them to not be used normally can vary widely, but here are some examples:
- the backup server is owned by a different company or organization
- the backup server does not have direct access to the primary mail storage
- the backup server cannot determine valid recipient addresses
- the backup server's Internet bandwidth costs more
- the backup server has significantly less Internet bandwidth
- the backup server has a high-latency Internet connection
Spammers
A favorite technique of spammers is to connect to the backup (high distance) MX servers for a domain first in order to avoid any anti-spam filters that may be running on the primary (lowest distance/highest preference) MX. Backup MX servers often have different anti-spam techniques, and using them can hide the spammer's IP address from the primary MX servers. This can be foiled by using bogus high-distance MX servers.
Alternatively, sometimes spammers only target the lowest-distance MX records for domains, and do not fall back to higher-distance MX records when the lowest-distance MX records are unreachable. A technique called nolisting will defeat this behavior.
