SMTP 521 Reply Code

Document Type Replaced Internet-Draft (individual)
Author John Klensin 
Last updated 2014-09-17 (latest revision 2014-08-11)
Replaced by RFC 7504
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Expired & archived
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IESG IESG state Replaced by draft-klensin-smtp-521code
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


This memo defines a new Simple Mail Transfer Protocol (SMTP) reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail. It is a standards track replacement for the earlier, experimental, RFC 1846 without any substantive changes.


John Klensin (

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)