mx.null_mx
Null MX (Domain Does Not Accept Email)
What we check
We check for a single MX record with priority 0 and target . (a single dot). This is the Null MX mechanism defined in RFC 7505 — a deliberate declaration that the domain does not accept email.
What this finding means
The domain has published a valid Null MX record. This is a positive signal: the domain has explicitly declared that it does not accept inbound email, and sending servers should not attempt delivery. Messages addressed to this domain are rejected immediately rather than queued and bounced later.
Why it matters
- Clear intent. A Null MX unambiguously communicates that no mail should be delivered, unlike a missing MX record which is ambiguous.
- Faster rejection. Sending servers that check MX records before connecting can reject messages immediately, avoiding the overhead of connection attempts, timeouts, and deferred bounces.
- Spoofing defense. Combined with
v=spf1 -allandv=DMARC1; p=reject, a Null MX completes the "this domain does not send or receive email" posture — useful for parked domains, asset-holding domains, or domains used only for web services.
Evidence example
$ dig +short MX example.com
0 .
Domain explicitly declares it does not accept email (RFC 7505 Null MX)