SMTP 521 and 556 Reply Codes
RFC 7504, “SMTP 521 and 556 Reply Codes”, is a Proposed Standard document published in June 2015 by J. Klensin. It updates RFC 1846, RFC 5321. The canonical text is published by the RFC Editor.
Abstract
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.
What “Proposed Standard” means
An entry-level standards-track specification: stable, peer-reviewed and a solid basis for implementation, though it may still evolve before becoming an Internet Standard.
The canonical text of RFC 7504 is hosted at rfc-editor.org. Available in TXT,HTML.
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