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RFC 7505 / NULL MX

Read the null MX record

02

One ordinary MX record uses preference 0 and the root label as its exchange name.

DIAGRAMRead RFC section 3 ↗
RFC 7505 · § 3EXAMPLE
3 / 3
Read the null MX recordA single master-file MX record for nomail.example has illustrative TTL 3600, class IN, preference 0 and exchange root dot. Its uncompressed RDATA is 00 00 00: a 16-bit zero preference and one zero-length root label. A separate invalid example mixes this with an ordinary MX 10 record and stops at the advertiser because other MX records MUST NOT be advertised with null MX. No recipient repair algorithm is invented.One MX record declares no exchangerSeparate invalid advertisementnomail.example. MX 0 .nomail.example. MX 10 mail.example.advertise bothMUST NOT advertise any other MXPublisher rule · no backup exchanger
03
Do not add a backup MX beside it.

Separate invalid advertisement: MX 0 . and MX 10 mail.example. for the same owner. The publisher MUST NOT advertise that combination.

Source for this drawing: RFC 7505 · Read RFC section 3 ↗

Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
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A message needs an address.

Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.

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RFC 5321 SMTP RFC 1035 DNS wire RFC 5322 Email message format

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Terms in this drawing
Owner
The domain name to which the DNS record belongs.
RDATA
The type-specific data inside a DNS resource record.
Root label
The zero-length final DNS label, written as a dot.
Related RFCs and changes

“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.

KEY POINT

The dot means no mail exchanger, not a server to contact.

  • A domain indicates no accepted email with a single MX record: preference 0 and an exchange consisting of the zero-length root label, written as a dot in master files.
  • A domain advertising a null MX MUST NOT advertise any other MX record. Do not mix it with ordinary or backup mail exchangers.
  • The illustrative record is nomail.example. 3600 IN MX 0 . . The 3600 second TTL is chosen for the example, not prescribed by this RFC. The record uses the existing MX type, not a new DNS type.
DETAILS

Read the last dot as the exchange name; the earlier trailing dot finishes the owner’s fully qualified name. For uncompressed RDATA, preference occupies two zero octets and the root label occupies one zero octet: 00 00 00. This is RDATA only, not the complete DNS response. Field boxes are schematic, not bit-width scale. Supporting RFC1035§§3.1,3.3.9 define label and MX encoding. The mixed-record state is a separate invalid advertisement; RFC7505 gives publishers this prohibition but does not define a repair algorithm for that invalid set.

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