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Simple Mail Transfer Protocol
RFC 821

Document type: RFC - Internet Standard (August 1982; No errata)
Obsoleted by RFC 2821
Obsoletes RFC 772, RFC 780, RFC 788
Document stream: Legacy
Last updated: 2013-03-02
Other versions: plain text, pdf, html

Legacy State: (None)
Document shepherd: No shepherd assigned

IESG State: RFC 821 (Internet Standard)
Responsible AD: (None)
Send notices to: No addresses provided

RFC 821
                                    
                                    
                                    
                                    
                                    
                     SIMPLE MAIL TRANSFER PROTOCOL
                                    
                                    
                                    
                           Jonathan B. Postel

                              August 1982
                                    
                                    
                                    
                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291

                             (213) 822-1511





                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol

                           TABLE OF CONTENTS

   1.  INTRODUCTION .................................................. 1

   2.  THE SMTP MODEL ................................................ 2

   3.  THE SMTP PROCEDURE ............................................ 4

      3.1.  Mail ..................................................... 4
      3.2.  Forwarding ............................................... 7
      3.3.  Verifying and Expanding .................................. 8
      3.4.  Sending and Mailing ..................................... 11
      3.5.  Opening and Closing ..................................... 13
      3.6.  Relaying ................................................ 14
      3.7.  Domains ................................................. 17
      3.8.  Changing Roles .......................................... 18

   4.  THE SMTP SPECIFICATIONS ...................................... 19

      4.1.  SMTP Commands ........................................... 19
      4.1.1.  Command Semantics ..................................... 19
      4.1.2.  Command Syntax ........................................ 27
      4.2.  SMTP Replies ............................................ 34
      4.2.1.  Reply Codes by Function Group ......................... 35
      4.2.2.  Reply Codes in Numeric Order .......................... 36
      4.3.  Sequencing of Commands and Replies ...................... 37
      4.4.  State Diagrams .......................................... 39
      4.5.  Details ................................................. 41
      4.5.1.  Minimum Implementation ................................ 41
      4.5.2.  Transparency .......................................... 41
      4.5.3.  Sizes ................................................. 42

   APPENDIX A:  TCP ................................................. 44
   APPENDIX B:  NCP ................................................. 45
   APPENDIX C:  NITS ................................................ 46
   APPENDIX D:  X.25 ................................................ 47
   APPENDIX E:  Theory of Reply Codes ............................... 48
   APPENDIX F:  Scenarios ........................................... 51

   GLOSSARY ......................................................... 64

   REFERENCES ....................................................... 67





Network Working Group                                          J. Postel
Request for Comments: DRAFT                                          ISI
Replaces: RFC 788, 780, 772                                  August 1982

                     SIMPLE MAIL TRANSFER PROTOCOL

1.  INTRODUCTION

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

   SMTP is independent of the particular transmission subsystem and
   requires only a reliable ordered data stream channel.  Appendices A,
   B, C, and D describe the use of SMTP with various transport services.
   A Glossary provides the definitions of terms as used in this
   document.

   An important feature of SMTP is its capability to relay mail across
   transport service environments.  A transport service provides an
   interprocess communication environment (IPCE).  An IPCE may cover one
   network, several networks, or a subset of a network.  It is important
   to realize that transport systems (or IPCEs) are not one-to-one with
   networks.  A process can communicate directly with another process
   through any mutually known IPCE.  Mail is an application or use of
   interprocess communication.  Mail can be communicated between
   processes in different IPCEs by relaying through a process connected
   to two (or more) IPCEs.  More specifically, mail can be relayed
   between hosts on different transport systems by a host on both
   transport systems.

Postel                                                          [Page 1]

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