MX

networking

Stands for: Mail Exchange record

The DNS record that says which servers receive a domain's email.

A Mail Exchange record (RFC 5321) lists the mail servers for a domain, each with a preference number so senders try the lowest-numbered server first.

An MX record says which hosts accept mail for a domain, and the number in front of each one is a preference where lower wins. It is one of the few record types whose semantics are genuinely about priority rather than address resolution, and equal values are how mail gets load balanced across several servers.

The common misunderstanding is that a backup MX with a higher number provides resilience. It provides an alternative destination, and if that host merely queues mail without the same anti-spam and routing configuration as the primary, it becomes the soft target attackers aim at deliberately. A backup that is weaker than the primary is a liability rather than redundancy.

MX no longer stands alone. SPF, DKIM and DMARC records in the same zone determine whether mail claiming to be from the domain is accepted anywhere, and a domain that sends no mail at all should still publish records saying so, because an unprotected domain is one attackers can impersonate freely. Mail configuration is therefore a DNS exercise as much as a server one.

Also known as: mx, MX record

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