RFC 7504: SMTP 521 and 556 Reply Codes
In plain English — editorial summary, not part of the RFC
This memo defines two Simple Mail Transfer Protocol (SMTP) reply codes, 521 and 556. The 521 code was originally described in an Experimental RFC in 1995 and is in wide use, but has not previously been formally incorporated into SMTP. The 556 code was created to support the new tests and actions specified in RFC 7505. These codes are used to indicate that an Internet host does not accept incoming mail at all. This specification is not applicable when the host sometimes accepts mail but may reject particular messages, or even all messages, under specific circumstances.
Document record
- Document ID
- RFC7504
- Published
- June 2015
- Authors
- J. Klensin
- Status
- PROPOSED STANDARD
- Stream
- IETF
- Area
- —
- Pages
- 7
- Also known as
- —
Topics
Related documents
Ranked automatically by shared keywords, IETF area and stream — not by editorial selection.
- RFC 7377IMAP4 Multimailbox SEARCH ExtensionCurrent
October 2014
- RFC 7293The Require-Recipient-Valid-Since Header Field and SMTP Service ExtensionCurrent
July 2014
- RFC 7200A Session Initiation Protocol (SIP) Load-Control Event PackageCurrent
April 2014
- RFC 8058Signaling One-Click Functionality for List Email HeadersCurrent
January 2017
- RFC 6785Support for Internet Message Access Protocol (IMAP) Events in SieveCurrent
November 2012
- RFC 8255Multiple Language Content TypeCurrent
October 2017
- RFC 8301Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)Current
January 2018
- RFC 6686Resolution of the Sender Policy Framework (SPF) and Sender ID ExperimentsCurrent
July 2012
Also filed under
About this page
The document record above — title, authors, date, status, stream, area, relationships, DOI and errata — is imported verbatim from the public RFC Editor index. The “in plain English” section is editorial: written by The metasystema editorial team, not part of the RFC. Where the two differ, the RFC text governs.
Last checked against the RFC Editor index on . RFCs are never revised after publication; changes are issued as new documents.
Data sources · Editorial policy · Report a correction · What is an RFC?