What is an MX record?

An MX (mail exchanger) record is a DNS record that tells sending mail servers which host or hosts accept email for a domain, each with a preference number. Senders connect to the host with the lowest number first and fall back to the others if it does not answer. MX records are defined in RFC 5321 and RFC 1035.

Example

example.com.  MX 10 mx1.spamjadoo-hosted.example.
example.com.  MX 20 mx2.spamjadoo-hosted.example.

A sender tries mx1 first. If the connection fails, it tries mx2. Two hosts with the same number share the load at random.

Rules that trip people up

• The target of an MX must be a hostname with an A or AAAA record, not an IP address and not a CNAME (RFC 2181 section 10.3).
• A domain with no MX still receives mail: senders fall back to the domain’s own A record. Publish an explicit MX so that behaviour is intentional.
• A single record of MX 0 . is a null MX (RFC 7505). It says the domain never accepts mail, which stops senders queuing and retrying for days.
• Backup MX hosts that accept mail and forward it later are a spam vector, because spammers target the backup to bypass checks on the primary. Every MX host should apply the same policy.

Where an anti-spam gateway sits

Deploying Spamjadoo means pointing the lowest-numbered MX at a Spamjadoo node. All inbound connections hit that node, go through the handshake checks, and only accepted mail is relayed to the real mail store. Use the MX lookup to see any domain’s current setup.

Last reviewed 10 September 2026 by Spamjadoo engineering.