MX host
A server named in a domain's MX records as a place to deliver that domain's mail; senders try the host with the lowest preference value first.
What it means
To deliver to ana@example.net, a sending server looks up the MX records of example.net. Each record gives a preference number and a host name; the sender tries the most preferred host (the lowest number) first and moves on if it cannot deliver there, and RFC 5321 requires clients to spread the load randomly across hosts of equal preference. Each MX host name must resolve to an address record; RFC 2181 forbids pointing an MX at an alias (CNAME).
If a domain has no MX records, the sender falls back to the domain's own address (the implicit MX), and a domain that takes no mail can publish a null MX. Which provider runs a domain's MX hosts tells a sender whose filtering a message will meet.
Example
example.net. 3600 IN MX 10 mx1.example.net.
example.net. 3600 IN MX 20 mx2.example.net.Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 5.1: Locating the Target Hostrfc-editor.org
- RFC 2181, section 10.3: MX and NS recordsrfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes