SMTP Service Extension for Secure SMTP over TLS
RFC 2487

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Network Working Group                                     P. Hoffman
Request for Comments: 2487                  Internet Mail Consortium
Category: Standards Track                               January 1999

            SMTP Service Extension for Secure SMTP over TLS

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Abstract

   This document describes an extension to the SMTP service that allows
   an SMTP server and client to use transport-layer security to provide
   private, authenticated communication over the Internet. This gives
   SMTP agents the ability to protect some or all of their
   communications from eavesdroppers and attackers.

2. Introduction

   SMTP [RFC-821] servers and clients normally communicate in the clear
   over the Internet. In many cases, this communication goes through one
   or more router that is not controlled or trusted by either entity.
   Such an untrusted router might allow a third party to monitor or
   alter the communications between the server and client.

   Further, there is often a desire for two SMTP agents to be able to
   authenticate each others' identities. For example, a secure SMTP
   server might only allow communications from other SMTP agents it
   knows, or it might act differently for messages received from an
   agent it knows than from one it doesn't know.

   TLS [TLS], more commonly known as SSL, is a popular mechanism for
   enhancing TCP communications with privacy and authentication. TLS is
   in wide use with the HTTP protocol, and is also being used for adding
   security to many other common protocols that run over TCP.

Hoffman                     Standards Track                     [Page 1]
RFC 2487                 SMTP Service Extension             January 1999

2.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC-2119].

3. STARTTLS Extension

   The STARTTLS extension to SMTP is laid out as follows:

   (1) the name of the SMTP service defined here is STARTTLS;

   (2) the EHLO keyword value associated with the extension is STARTTLS;

   (3) the STARTTLS keyword has no parameters;

   (4) a new SMTP verb, "STARTTLS", is defined;

   (5) no additional parameters are added to any SMTP command.

4. The STARTTLS Keyword

   The STARTTLS keyword is used to tell the SMTP client that the SMTP
   server allows use of TLS. It takes no parameters.

5. The STARTTLS Command

   The format for the STARTTLS command is:

   STARTTLS

   with no parameters.

   After the client gives the STARTTLS command, the server responds with
   one of the following reply codes:

   220 Ready to start TLS
   501 Syntax error (no parameters allowed)
   454 TLS not available due to temporary reason

   A publicly-referenced SMTP server MUST NOT require use of the
   STARTTLS extension in order to deliver mail locally. This rule
   prevents the STARTTLS extension from damaging the interoperability of
   the Internet's SMTP infrastructure. A publicly-referenced SMTP server
   is an SMTP server which runs on port 25 of an Internet host listed in
   the MX record (or A record if an MX record is not present) for the
   domain name on the right hand side of an Internet mail address.

Hoffman                     Standards Track                     [Page 2]
RFC 2487                 SMTP Service Extension             January 1999

   Any SMTP server may refuse to accept messages for relay based on
   authentication supplied during the TLS negotiation. An SMTP server
   that is not publicly referenced may refuse to accept any messages for
   relay or local delivery based on authentication supplied during the
   TLS negotiation.

   A SMTP server that is not publicly referenced may choose to require
   that the client perform a TLS negotiation before accepting any
   commands. In this case, the server SHOULD return the reply code:

   530 Must issue a STARTTLS command first

   to every command other than NOOP, EHLO, STARTTLS, or QUIT. If the
   client and server are using the ENHANCEDSTATUSCODES ESMTP extension
   [RFC-2034], the status code to be returned SHOULD be 5.7.0.

   After receiving a 220 response to a STARTTLS command, the client
   SHOULD start the TLS negotiation before giving any other SMTP
   commands.

   If the SMTP client is using pipelining as defined in RFC 1854, the
   STARTTLS command must be the last command in a group.

5.1 Processing After the STARTTLS Command

   After the TLS handshake has been completed, both parties MUST