MX Records and Mail Routing: Priorities, Backup MX, and Null MX Explained

MX (Mail Exchanger) records tell sending servers which servers accept email for your domain, and in what order of preference. They are the backbone of email routing, and a few details — priorities, backup MX, and Null MX — trip people up. Here is how they really work. To configure them, see configuring MX records.

How MX priorities work

Each MX record has a priority number, and lower numbers mean higher preference. A sending server tries the lowest-numbered MX first; if it cannot connect, it moves to the next. So an MX at priority 10 is tried before one at priority 20.

yourdomain.com. MX 10 mail1.yourdomain.com.
yourdomain.com. MX 20 mail2.yourdomain.com.

Multiple MX and load spreading

Give two records the same priority and sending servers distribute between them, spreading load across mail servers. Different priorities instead create a clear primary and fallback order.

Backup MX: what it really does

A backup MX is a lower-preference (higher-number) mail server that accepts mail when your primary is down, holds it, and forwards it once the primary returns. It reduces the chance of a bounce during an outage — but a well-run primary with retrying senders often makes it unnecessary, since sending servers already retry for days. If you do run one, it must be configured to relay only for your domain, or it becomes an open relay risk.

Null MX: "we accept no mail"

If a domain never receives email, publishing a Null MX — a single MX record with priority 0 pointing to . (a lone dot) — explicitly says "this domain accepts no mail". Sending servers then reject undeliverable mail immediately instead of retrying for days. It is the correct way to mark a send-only or parked domain.

MX points to names, not IPs

An MX must point to a hostname with its own A/AAAA record — never directly to an IP, and never to a CNAME. Verify your MX with dig MX, and remember mail also needs reverse DNS and SPF/DKIM/DMARC to deliver well.

Frequently asked questions

Does a lower MX priority number mean less important?

The opposite — lower numbers are tried first, so they are the preferred servers. A priority 10 MX is used before a priority 20 one. Think of the number as "distance": nearer (lower) is preferred.

Do I need a backup MX?

Usually not — sending servers already retry for days if your primary is briefly down. A backup MX only helps in prolonged outages, and it must be properly restricted to your domain to avoid being abused as a relay.

What is a Null MX for?

To declare that a domain receives no email, so senders reject mail to it immediately rather than retrying. Use it on send-only or parked domains — it is a single MX priority 0 pointing to a dot.