Mail Transfer Protocol
RFC 772

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Network Working Group                                         S. Sluizer
Request for Comments: 772                                      J. Postel
                                                                     ISI
                                                          September 1980

                         MAIL TRANSFER PROTOCOL

PREFACE

   This is a first draft of this protocol and comments are very
   definitely requested.

INTRODUCTION

   The objective of Mail Transfer Protocol (MTP) is to transfer mail
   reliably and efficiently.

   This paper assumes knowledge of the following protocols described in
   the ARPA Internet Protocol Handbook.  The reader will note strong
   similarities to portions of the File Transfer Protocol; in part, this
   is due to the original ARPA Network implementation of computer mail
   as a feature of FTP.

      The ARPANET Host-to-Host Protocol [Network Control Protocol] (NCP)

      The Transmission Control Protocol (TCP)

      The TELNET Protocol (TELNET)

      The File Transfer Protocol (FTP)

DISCUSSION

   In this section, the terminology and the MTP model are discussed.
   The terms defined in this section are only those that have special
   significance in MTP.  Some of the terminology is very specific to the
   MTP model; some readers may wish to turn to the section on the MTP
   model while reviewing the terminology.

   TERMINOLOGY

      ASCII

         The ASCII character set as defined in the ARPA Internet
         Protocol Handbook.  In MTP, ASCII characters are defined to be
         the lower half of an eight-bit code set (i.e., the most
         significant bit is zero) and is called NVT-ASCII.

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September 1980                                                   RFC 772
Mail Transfer Protocol                                                  

      control connection

         The TCP full-duplex communication path or two NCP simplex
         communication paths between a sender-MTP and a receiver-MTP for
         the exchange of commands, replies, and mail text.  The control
         connection operates according to the TELNET Protocol.

      data mode

         The mail is transmitted over the control connection as a stream
         of octets.  (In FTP terminology this is called stream mode.)

      data structure

         The internal structure of mail is considered to be a continuous
         sequence of data octets.  (In FTP terminology this is called
         file-structure.)

      data representation

         The internal representation of all data (i.e., mail) is in
         NVT-ASCII.

      host

         A computer in the internetwork environment on which mailboxes
         reside.

      MTP commands

         A set of commands which comprise the control information
         flowing from the sender-MTP to the receiver-MTP.

      mail

         An ordered set of computer data of arbitrary length, which
         conforms to the standard set in RFC 733 (Standard for the
         Format of ARPA Network Text Messages).

      mailbox

         A character string (address) which identifies a user to whom
         mail is to be sent.  Mailbox normally consists of the host and
         user specifications.  The standard mailbox naming convention is
         defined to be "user@host".  Additionally, the "container" in
         which mail is stored.

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RFC 772                                                   September 1980
                                                  Mail Transfer Protocol

      NVT

         The Network Virtual Terminal as defined in the TELNET Protocol.

      octet

         Bytes in MTP are octets (8 bits).  This is not necessarily the
         same byte size in which data is stored in a host.

      reply

         A reply is an acknowledgment (positive or negative) sent from
         receiver to sender via the control connection in response to a
         MTP command.  The general form of a reply is a completion code
         (including error codes) followed by a text string.  The codes
         are for use by programs and the text is usually intended for
         human users.

      receiver-MTP process

         A process which transfers mail in cooperation with a sender-MTP
         process.  It "listens" on its port/socket L for a connection
         from a sender-MTP and establishes a control connection using
         the TELNET Protocol.  It receives MTP commands from the
         sender-MTP, sends replies, and governs the transfer of mail.

      sender-MTP process

         A process which transfers mail in cooperation with a
         receiver-MTP process.  A local language may be used in the user
         interface command/reply dialogue.  The sender-MTP initiates the