Records

MX: mail exchange

MX records tell a sending SMTP server where to deliver mail for a domain. Each MX carries a numeric preference; the lowest number is tried first.

run it right here; the dig tool has every option

open in the dig tool →

typeMXwire type15rfc1035, 5321common ttl300

In one line:

example.com.    MX    10    mx1.mailprovider.com.
└── mail for ┘ └type┘ └#┘   └── goes to this server (lowest # first)

What it holds#

A preference number (16 bits on the wire) and a hostname the sender resolves next for A/AAAA. The hostname cannot be a CNAME (RFC 2181 §10.3). Lowest preference is tried first; equal preferences tie-break at the sender's discretion (RFC 5321 §5.1).

mx.example.isitdns.net.   MX  10  mx.example.com.

When to use it#

For any domain that receives mail. The apex is the typical place, but you can have MX at any name (for departmental routing, e.g., sales.example.com).
For backup MX in front of an on-prem mail server: a higher-preference MX at a hosted service that queues during outages.

When not to use it#

For a domain that doesn't send or receive mail: publish a null MX instead (see Gotchas). Easier than fighting senders that keep retrying for days.
For load balancing: preference is a discrete ranking, not a weighted load distribution. Use equal-preference for round-robin instead.

dig example#

Asks which servers take mail for mx.example.isitdns.net.

$ dig @1.1.1.1 mx.example.isitdns.net MX

; <<>> DiG 9.20.11-0ubuntu0.2-Ubuntu <<>> @1.1.1.1 mx.example.isitdns.net MX

; (1 server found)

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47825

;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:

; EDNS: version: 0, flags:; udp: 1232

;; QUESTION SECTION:

;mx.example.isitdns.net. IN MX

;; ANSWER SECTION:

mx.example.isitdns.net. 300 IN MX 10 mx.example.com.

;; Query time: 36 msec

;; SERVER: 1.1.1.1#53(1.1.1.1) (UDP)

;; WHEN: Mon Sep 14 13:45:34 CDT 2026

;; MSG SIZE rcvd: 81

GO NOERROR, one MX record, preference 10

NO GO NOERROR with no answer, which means senders fall back to this name's A/AAAA

run this query in the dig tool →

what each part of this output means →

RFC: RFC 1035 defines the MX record; RFC 5321 §5.1 defines how a sender uses it.

Gotchas#

The MX target must resolve to an A or AAAA record, not a CNAME (RFC 2181 §10.3) or a bare IP.
No MX does not mean no mail. With no MX at all, senders fall back to an implicit MX: the domain's own A/AAAA record, treated as a mail server with preference 0 (RFC 5321 §5.1).
Null MX for non-mail domains: RFC 7505 says a domain that never receives mail should publish a single MX record with preference 0 and a target of . (literal dot):

example.com.   MX   0   .

Senders interpret this as "do not deliver here" and bounce immediately rather than retrying for days.

SPF/DKIM/DMARC are separate: having an MX does not authorize anyone to send on your behalf. See email-records.