What is an MX record? A plain explanation with examples

An MX record is the line in your domain’s DNS that says where email for that domain should be delivered. Get it wrong and mail does not bounce right away: it waits, retries, and then disappears days later.

MX records in one paragraph

MX stands for mail exchanger. Each MX record has two parts: a priority number and the hostname of a mail server. When someone sends mail to you@example.com, their server asks DNS for the MX records of example.com, picks the one with the lowest number, looks up that hostname’s IP address and delivers the message there.

A real MX record example

Here is a small but complete mail setup for a domain that runs its own server:

example.com.        3600  MX   10 mail.example.com.
example.com.        3600  MX   20 backup.example.net.
mail.example.com.   3600  A    203.0.113.10
mail.example.com.   3600  AAAA 2001:db8::10
example.com.        3600  TXT  "v=spf1 mx ~all"

Reading it left to right: the domain, the TTL in seconds, the record type, the priority, and the target hostname. The trailing dot means "this is a full name", and most DNS panels add it for you.

For a hosted provider the MX record is usually a single line. Google Workspace uses 1 smtp.google.com., and Microsoft 365 uses a host under mail.protection.outlook.com. Our guides for Google Workspace and Microsoft 365 list the exact records.

How sending servers use MX records

1. The sender’s server looks up the MX records for the domain after the @.
2. It sorts them by priority, lowest number first. See how MX priority works.
3. It resolves the chosen hostname to an IPv4 (A) or IPv6 (AAAA) address.
4. It opens an SMTP connection on port 25, usually upgrades to TLS with STARTTLS, and hands over the message.
5. If that server does not answer, it tries the next MX. If none answer, it queues the message and retries for several days.

If a domain has no MX record at all, senders fall back to the domain’s own A record. That fallback is why a parked domain with a web server can still receive spam attempts on port 25.

The target must be a hostname with an A record

This is the rule people break most often. An MX record points to a name, not a number, and that name must resolve with its own A or AAAA record.

Null MX: saying "this domain takes no mail"

Domains that never receive email, such as a parked domain or a site that only sends notifications from a subdomain, should publish a null MX as defined in RFC 7505:

example.org.   3600  MX   0 .
example.org.   3600  TXT  "v=spf1 -all"
_dmarc.example.org. 3600 TXT "v=DMARC1; p=reject"

The single dot means "no mail server". Senders bounce at once instead of retrying for days, and with the SPF and DMARC lines nobody can send mail as that domain either. A null MX must be the only MX record for the domain.

Forwarding-only MX records

Many registrars offer free email forwarding. They do it by setting your MX records to their own forwarding servers, for example 10 mx1.forwarding.example-registrar.com.. Mail arrives there and is passed on to your Gmail or iCloud address.

That is fine for receiving. Two things to know: forwarded mail often fails SPF at the final inbox, and you cannot send replies through the forwarder. If you reply from Gmail as contact@yourdomain, the message is not covered by your domain’s SPF or DKIM unless you set up proper sending.

Common MX record mistakes

• Leftover records from an old host. After a move, the old MX stays with a lower number and keeps getting mail you never see.
• An IP address in the MX field. Use a hostname and add an A record for it.
• MX pointing to a CNAME. Point it at the real hostname instead.
• Hostname in the name field. In most panels the name is @ (the domain itself), not mail.
• Missing trailing dot in a zone file. mail.example.com without the dot becomes mail.example.com.example.com.
• A backup MX with weaker filtering. Spammers target it on purpose. See MX priority and backup MX.
• No MX on a domain that sends mail. Replies and bounces have nowhere to go, and some receivers score the sender lower.

Check your MX records

Run a single domain through the MX lookup to see its records, priorities and the IP behind each host. To check MX together with SPF, DKIM, DMARC, the STARTTLS certificate and blacklists for up to 25 domains at once, use the free mail health checker.

Questions