IETF RFC 7504 PDF

Introduction

The SMTP specification [2] (referred to, along with its various updates, as "SMTP" below) contains a list and discussion of reply codes. This document updates that list with a new code, 521, for use in response to an initial connection. In that context, it specifically denotes a system that does not receive mail or otherwise handle SMTP mail or inquiry transactions. That code differs from the use of reply code 554, recommended by RFC 5321, because the latter code can be used in a larger variety of situations, including mail that is not accepted for, or from, particular sources, destinations, or addresses. It also introduces a second reply code, 556, for use when an SMTP client encounters a domain in a forward-pointing address that it can determine (e.g., from the DNS "null MX" convention [5]) does not support receipt of mail and has to report that condition to a host that delivered the message to it for further processing.

This specification updates RFC 5321 to add the new codes. The 521 code was first formally proposed in the Experimental RFC 1846 [4]; this document updates that specification to standardize the code and provide more specific treatment of it.