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MX record

MX records tell other servers where to deliver email for a domain. Priorities, Google Workspace and Microsoft 365 values, null MX, how to check them and what breaks.

What it is

An MX record (mail exchanger) names the servers that accept email for a domain. When someone sends mail to you@example.com, their server looks up the MX records for example.com, picks the one with the lowest preference number and connects to it over SMTP. MX is defined in RFC 1035, and the delivery rules are in RFC 5321.

example.com.  3600  IN  MX  10 mx1.example.com.
example.com.  3600  IN  MX  20 mx2.example.com.

The fields

Field Example Meaning
Name example.com. The domain mail is addressed to (the part after @)
TTL 3600 Seconds resolvers may cache it
Type MX Mail exchanger
Preference 10 Lower is tried first; equal values share the load
Exchange mx1.example.com. Hostname of the mail server

The exchange must be a hostname with its own A or AAAA record. The field holds a domain name, so an IP address typed there is treated as a (nonexistent) name, and RFC 2181 forbids pointing MX at a CNAME. If a domain has no MX at all, senders fall back to its A/AAAA record (the "implicit MX"), which is rarely what you want.

Common setups

  • Google Workspace: a single record, MX 1 smtp.google.com. Older setups use five: aspmx.l.google.com (1), alt1 and alt2.aspmx.l.google.com (5), alt3 and alt4.aspmx.l.google.com (10). Both still work.
  • Microsoft 365: one record, MX 0 example-com.mail.protection.outlook.com., where the first label is your domain with dots replaced by dashes; copy the exact value from the admin center.
  • Domains that never receive mail: publish a null MX, example.com. MX 0 . (RFC 7505), so senders bounce immediately instead of retrying for days.
  • A filtering service in front of your mailbox: the MX points at the filter, which relays to your real provider.

MX only covers receiving. Sending is authenticated by SPF, DKIM and DMARC records.

How to check it

Use our DNS Lookup, or check MX, SPF, DKIM and DMARC together with the Email Checker. From a terminal:

dig example.com MX +short
nslookup -type=MX example.com

After switching providers, the DNS Propagation Checker shows whether public resolvers still hand out the old servers.

Common mistakes

  • MX on the wrong name. Mail for you@example.com uses the MX of example.com, not mail.example.com or www.
  • Old and new provider mixed. Leftover records from the previous host at a lower number keep receiving part of your mail. Delete them.
  • Pointing at an IP or a CNAME. Some senders tolerate it, many don't.
  • A missing trailing dot in a zone file, which turns mx1.example.com into mx1.example.com.example.com.
  • Changing MX with a long TTL. Mail keeps going to the old server until caches expire; lower the TTL beforehand and keep the old mailbox running for a while.