Local Mail Transfer Protocol
RFC 2033

Document Type RFC - Informational (October 1996; No errata)
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Network Working Group                                           J. Myers
Request for Comments: 2033                               Carnegie Mellon
Category: Informational                                     October 1996

                      Local Mail Transfer Protocol

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

1.  Abstract

   SMTP [SMTP] [HOST-REQ] and its service extensions [ESMTP] provide a
   mechanism for transferring mail reliably and efficiently.  The design
   of the SMTP protocol effectively requires the server to manage a mail
   delivery queue.

   In some limited circumstances, outside the area of mail exchange
   between independent hosts on public networks, it is desirable to
   implement a system where a mail receiver does not manage a queue.
   This document describes the LMTP protocol for transporting mail into
   such systems.

   Although LMTP is an alternative protocol to ESMTP, it uses (with a
   few changes) the syntax and semantics of ESMTP.  This design permits
   LMTP to utilize the extensions defined for ESMTP.  LMTP should be
   used only by specific prior arrangement and configuration, and it
   MUST NOT be used on TCP port 25.

Table of Contents

   1.   Abstract ................................................    1
   2.   Conventions Used in this Document .......................    2
   3.   Introduction and Overview ...............................    2
   4.   The LMTP protocol .......................................    3
   4.1. The LHLO, HELO and EHLO commands ........................    4
   4.2. The DATA command ........................................    4
   4.3. The BDAT command ........................................    5
   5.   Implementation requirements .............................    6
   6.   Acknowledgments .........................................    6
   7.   References ..............................................    7
   8.   Security Considerations .................................    7
   9.   Author's Address ........................................    7

Myers                        Informational                      [Page 1]
RFC 2033                          LMTP                      October 1996

2.  Conventions Used in this Document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

3.  Introduction and Overview

   The design of the SMTP protocol effectively requires the server to
   manage a mail delivery queue.  This is because a single mail
   transaction may specify multiple recipients and the final "." of the
   DATA command may return only one reply code, to indicate the status
   of the entire transaction.  If, for example, a server is given a
   transaction for two recipients, delivery to the first succeeds, and
   delivery to the second encounters a temporary failure condition,
   there is no mechanism to inform the client of the situation.  The
   server must queue the message and later attempt to deliver it to the
   second recipient.

   This queuing requirement is beneficial in the situation for which
   SMTP was originally designed: store-and-forward relay of mail between
   networked hosts.  In some limited situations, it is desirable to have
   a server which does not manage a queue, instead relying on the client
   to perform queue management.  As an example, consider a hypothetical
   host with a mail system designed as follows:

                    TCP port 25 +-----------------+
         ---------------------->|                 |  #########
                                |      Queue      |<># Mail  #
         TCP port 25            |     Manager     |  # Queue #
         <----------------------|                 |  #########
                                +-----------------+
                            Local *  ^ Local    * Local
                              IPC *  | IPC      * IPC
                                  *  |          *
                                  *  |          *
                                  *  |          *
                                  V  |          V
                  Non-SMTP    +----------+     +----------+
                  Protocol    | Gateway  |     |  Local   |  #########
              <==============>| Delivery |     | Delivery |>># Mail  #
                              |  Agent   |     |  Agent   |  # Spool #
                              +----------+     +----------+  #########

   The host's mail system has three independent, communicating
   subsystems.  The first is a queue manager, which acts as a

Myers                        Informational                      [Page 2]
RFC 2033                          LMTP                      October 1996

   traditional SMTP agent, transferring messages to and from other hosts
   over TCP and managing a mail queue in persistent storage.  The other
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