mx.null_mx_invalid
Null MX Coexists With Other MX Records
What we check
We check whether a Null MX record (priority 0, target .) is the domain's only MX record. RFC 7505 §3 requires the Null MX to be the sole MX record — it cannot coexist with other MX records.
What this finding means
The domain publishes a Null MX (declaring it does not accept email) alongside one or more regular MX records (pointing to mail servers). This is a contradictory configuration: the domain simultaneously claims it does not accept mail and provides servers to accept mail.
RFC 7505 §3 is explicit: a Null MX must be the only MX record. When other records coexist, the Null MX is malformed, and the behavior is undefined — different sending servers will interpret this differently.
Why it matters
- Undefined behavior. Some senders will honour the Null MX and refuse delivery; others will ignore it and deliver to the regular MX targets. The domain gets inconsistent mail delivery.
- Likely a migration error. This usually happens when a Null MX is added to stop mail delivery but the old MX records are not removed.
- Invalid configuration. The conflicting records indicate a DNS zone that has not been properly maintained.
How to fix
If the domain should accept email, remove the Null MX record and keep only the real MX records:
; Remove: example.com. IN MX 0 . ; Keep: example.com. IN MX 10 mail.example.com.If the domain should NOT accept email, remove the regular MX records and keep only the Null MX:
; Keep only: example.com. IN MX 0 .Verify:
dig +short MX example.com # Expected: either "0 ." alone, or real MX targets — never both
How it's graded
A Null MX coexisting with other records carries a 15-point deduction in the MX category. See Grading Methodology for the full scoring model.
Evidence example
$ dig +short MX example.com
0 .
10 mail.example.com.
Null MX coexists with a real MX target — contradictory configuration (RFC 7505 §3)