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What is an MX record?

Last updated .

An MX (mail exchanger) record names a server that accepts email for a domain, together with a priority. A sender looks up the MX records of the recipient’s domain and tries the server with the lowest priority number first, then the others.

Definition
MX record
A DNS record that names the mail servers accepting email for a domain, each with a priority number.

What breaks

If the MX records are deleted, senders have nowhere to deliver and messages bounce or are delayed. If they point to a former provider, mail goes to a mailbox no one reads. A website DNS change that replaces the zone or the nameservers is the usual way MX records are lost.

How it fits with authentication

MX decides where mail is delivered. SPF, DKIM and DMARC decide whether mail sent as the domain is trusted. A migration that changes one should be checked for the others.

An example

example.com. 3600 IN MX 10 mx1.mailhost.example. and example.com. 3600 IN MX 20 mx2.mailhost.example.: senders try mx1 first, because 10 is the lower number, and mx2 only if the first is unreachable. dig MX example.com +short lists them. An MX that points at the website’s own address, left over from an old setup, is a sign that mail may be delivered to a server that runs no mail service.

After any change of mail provider, send a test message from an outside account to the domain and one back out, then read the headers of both: delivery shows the MX records are right, and the authentication results show that SPF, DKIM and DMARC still hold.

What to check

The list of MX records and their priorities against the mailbox provider’s current instructions, and a real test message sent from outside to the domain.

Sources

Read on . Standards and vendor pages change; confirm the current text before you rely on it.

Guides that use this term

  • SPFSPF is a DNS record listing the servers allowed to send email for a domain. The one-record rule, the ten-lookup limit and the mistakes that make mail fail.
  • DKIMDKIM adds a cryptographic signature to outgoing email, verified with a public key in DNS. Where the key lives and what happens when the selector record is removed.
  • DMARCDMARC is a DNS policy that tells receivers what to do with email that fails SPF and DKIM alignment: nothing, quarantine or reject. Why strict policies amplify DNS mistakes.
  • DNS recordA DNS record is one entry in a domain’s zone that maps a name to data such as an address, a mail server or a text value. The common types and what breaks when one changes.

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