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Null MX

published · September 10, 2026

A null MX record is a single MX record with preference 0 pointing at the root, which is how a domain states in DNS that it accepts no mail at all.

A null MX record is a single MX record with preference 0 pointing at the root — 0 . — which is how a domain states in DNS that it accepts no mail at all. It was standardised in RFC 7505 so that a sending server could stop trying immediately rather than queueing and retrying for days.

What Spaw answers

mx_found is false and the reason is null_mx. The verdict is undeliverable, which is the strongest thing the email product says: the domain's own operator has published that nothing can be delivered there.

Undeliverable answers are free, so a list full of null-MX domains costs nothing to clean.

Why it matters more than an absent record

An absent MX record is ambiguous — it might mean the operator forgot, and RFC 5321 says to fall back to the A record. A null MX is a deliberate statement. Domains used only for a website, or parked, increasingly publish one, which is why it is worth distinguishing from implicit MX rather than folding both into "no mail server".

Related

markdown version: /glossary/null-mx.md

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