mx

MX Record

In short: A DNS record type that defines which mail server accepts emails for a domain.

In more detail: An MX record consists of a priority and the hostname of the target server. With several MX records, a sending mail server first tries the one with the lowest priority number, and the next ones if that fails — this enables fault tolerance across several mail servers.

In Depth

emzett-digital.com.   3600   IN   MX   10   mail1.emzett-digital.com.
emzett-digital.com.   3600   IN   MX   20   mail2.emzett-digital.com.

Here, mail1 with priority 10 is preferred; only if this server is unreachable does the sending mail server try mail2 with priority 20. A lower number means higher priority — a common pitfall for beginners, who intuitively equate a higher number with “more important”.

An important technical special case: the target value of an MX record itself has to be a hostname again, resolvable to an IP address via an A/AAAA record — according to the standard, an MX record may not point directly to an IP address or a CNAME. If the MX record for a domain is completely missing, it counts as unreachable for mail purposes, even if the domain itself (via an A record) has a working website — website and mail deliverability are configured completely independently of each other via separate DNS record types.

See also: DNS Records, Mail Server, Mail