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RFC 7505 / NULL MX

Reject an undeliverable recipient promptly

03

A submission or relay server can detect null MX while processing a recipient and report permanent failure without a target connection.

DIAGRAMRead RFC section 4.1 ↗
RFC 7505 · § 4.1EXAMPLE
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Reject an undeliverable recipient promptlyAn SMTP client sends RCPT TO alice at nomail.example to an existing submission or relay server. The relay asks DNS, receives a single MX 0 root, and rejects the recipient without opening a target SMTP connection. The recommended recipient-domain reply pair is 556 and5.1.10, not the sender-policy pair 550 and 5.7.27. The code choice is SHOULD, not MUST.The relay can reject before target deliverySMTP clientRelayDNS resolverRCPT TORCPT TO:<alice@nomail.example>MX lookupMX 0 .No target SMTP connection is opened556 5.1.10SHOULD userecipient error
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Return the recipient-domain failure.

The relay rejects that recipient. The recommended pair is 556 5.1.10; the message is not queued for repeated delivery attempts to this no-service domain.

Source for this drawing: RFC 7505 · Read RFC section 4.1 ↗

Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
New to this? Start hereA quick picture + helpful background

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.

Helpful first, if these ideas are unfamiliar

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

These pictures are analogies to help you learn. The linked RFC explains the actual rules.

Terms in this drawing
RCPT TO
The SMTP command specifying an envelope recipient.
Relay
An SMTP system that handles mail for onward delivery.
Enhanced status
A dotted status code giving a more specific reason than the basic reply.
Related RFCs and changes

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

KEY POINT

The relay can reject the recipient before dialing the destination.

  • Null MX allows delivery failure to be reported on the first attempt, avoiding pointless queuing and retries for that no-service destination.
  • When a submission or SMTP relay server rejects an envelope recipient because the destination domain has null MX, it SHOULD use reply 556 and enhanced status 5.1.10.
  • 556 means the domain does not accept mail; 5.1.10 identifies recipient-address null MX. This is different from rejection because a sender address has null MX.
DETAILS

The example RCPT command is sent to a submission/relay server, not to a nonexistent mail server at nomail.example. Supporting RFC7504§4 explains 556 as a permanent negative completion. The wording is SHOULD for the code choice, not an invented MUST that every implementation emits identical text. This error does not prove that a mailbox’s local-part is misspelled.

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