MX record
The DNS record that names the servers accepting mail for a domain, each with a preference number; senders try the lowest number first.
What it means
An MX record holds a preference and a host name: 10 mx1.example.com. Senders look up a domain's MX records, sort them with the lowest preference first, and try each until one accepts. Records with equal preference share the load.
The host name must have A or AAAA records and must not be an IP address or a CNAME. If a domain has no MX records at all, senders fall back to its address records (the implicit MX rule). A domain that accepts no mail can publish a null MX instead.
MX records control where mail arrives, not where it is sent from. Outbound servers are authorized through SPF, which is why the SPF mx mechanism only helps if the same servers send.
Example
example.com. 3600 IN MX 10 mx1.example.com.
example.com. 3600 IN MX 20 mx2.example.com.Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes