From LQWiki

An MX record (mail exchange record) is a record in a DNS system specifying to which server mail should be routed.

For example the MX record of the domain linuxquestions.org is mail.linuxquestions.org:

$ dig linuxquestions.org -t MX

; <<>> DiG 9.3.2 <<>> linuxquestions.org -t MX
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 64759
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; QUESTION SECTION:
;linuxquestions.org.            IN      MX

;; ANSWER SECTION:
linuxquestions.org.     14400   IN      MX      10 mail.linuxquestions.org.

;; ADDITIONAL SECTION:
mail.linuxquestions.org. 14400  IN      A       64.179.4.148

;; Query time: 139 msec
;; SERVER: 217.237.150.51#53(217.237.150.51)
;; WHEN: Tue Aug  7 20:52:17 2007
;; MSG SIZE  rcvd: 73

If you have an eMail for the address someone@linuxquestions.org, linuxquestions.org is the recipient's domain. This domain's MX record tells you which server from this domain is an SMTP server. In our case, it is mail.linuxquestions.org.

See also

• dig
• mail
• http://en.wikipedia.org/wiki/MX_record