What does MX stand for?
MX stands for Mail Exchange. The DNS record that names the mail server or servers responsible for a domain, each with a priority value where lower is preferred. Sending servers look up the MX record to deliver mail; a missing or wrong MX means inbound email bounces.
An MX record tells the world which servers accept email for a domain. When a server has a message for someone at example.com, it asks DNS for the MX records of example.com, receives one or more host names, each with a preference number, and tries them starting with the lowest number. Records with the same preference share the load. If a domain has no MX record at all, the sender falls back to the domain's own address record. A domain that receives no mail can say so with a null MX: a single record with preference 0 and a dot in place of the host.
MX in practice: an example
A company moves its mail to a hosted service, and the provider gives it two host names. The administrator publishes one MX record with preference 10 for the first and one with preference 20 for the second. Senders deliver to the first. When it is down for maintenance they retry on the second, and no mail is lost.
What MX is confused with
MX vs A record: what is the difference?
An A record gives the IP address of a host name. An MX record gives the host name of a mail server, and the sender then looks up that name's address record.
MX vs SPF: what is the difference?
MX is about receiving: where to deliver mail for the domain. SPF is about sending: which servers may send mail that claims to come from the domain. They are separate records and a domain needs both. What SPF stands for
Which exams cover MX?
MX is on the official CompTIA acronym list for two of the four exams covered here.
- Network+ N10-009: 3.0 (Network Operations).
- A+ 220-1201 and 220-1202: 220-1201 2.0 (Networking).
Each number is a CompTIA exam objective and the name in brackets is its domain. The exam puts the term in a scenario; it does not ask you to recite the expansion.
Check yourself: two practice questions
Written by CertSprint for this page. They are not taken from any exam.
1. A domain has MX records with preferences 10, 20 and 30. Which server does a sending mail server try first?
- The one with preference 10
- The one with preference 30
- Whichever answers DNS fastest
- All three at the same time
Show the answer
A. The one with preference 10. Lower preference numbers are tried first. The higher numbers are fallbacks for when the preferred server cannot be reached.
2. After a DNS migration, a company can send email but nothing arrives from outside. Its website and other records work. What should be checked first?
- The domain's MX records
- The DHCP scope
- The wireless network name
- The mail client's default gateway
Show the answer
A. The domain's MX records. Inbound delivery depends on senders finding the domain's mail servers through its MX records. Sending out does not use the domain's own MX, which is why it still works.
Sources: RFC 5321: Simple Mail Transfer Protocol; RFC 1035: Domain Names - Implementation and Specification; RFC 7505: A Null MX No Service Resource Record for Domains That Accept No Mail. The acronym and its expansion come from CompTIA's official acronym lists, linked from each exam glossary above. The explanation, example and questions are CertSprint's own, checked against these sources on 12 November 2026.