Glossary
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Mail transport & reputation

What is MX? Mail Exchanger record explained

MX records define which servers accept email for a domain. A domain that receives no mail should say so explicitly with a null MX record.

How MX works

An MX record consists of a priority and a host name. Sending servers try the entry with the lowest number first and the next one if it fails. Several MX records therefore provide redundancy.

The target must be a host name with an A or AAAA record – not an IP address and not a CNAME. The mail server should have a public IP and matching reverse DNS.

If a domain has no MX at all, senders try to deliver to the IP of the A record. A domain that receives no mail therefore publishes a null MX ("0 .", RFC 7505): senders then know immediately that mail to this domain cannot be delivered.

Example

example.com.  MX  10 mx1.example.com.
example.com.  MX  20 mx2.example.com.

; domain without inbound mail:
example.org.  MX  0 .

mx1 takes precedence, mx2 steps in if it fails. example.org accepts no mail.

Common mistakes

  • The MX points to a CNAME or directly to an IP address.
  • An MX host no longer resolves, for example after switching providers.
  • Private IP addresses (e.g. 192.168.x.x) in the mail server's A record.
  • Domains without inbound mail lack a null MX – senders then try the web server.

How the Domain Trust Check rates MX

Criterion “MX configuration” (weight 5 of 5): MX records present or null MX (RFC 7505), all MX hosts resolvable, no CNAME, public IP addresses, matching reverse DNS (PTR), redundancy and IPv6.

Standard: RFC 5321, RFC 7505 (null MX)

Related terms