Introduction

Memos in the Requests for Comments (RFC) document series contain technical and organizational notes about the Internet.

They cover many aspects of computer networking, including protocols, procedures, programs, and concepts, as well as meeting notes, opinions, and sometimes humor.

Information about all RFCs can be accessed on https://www.ietf.org/rfc.html

Some Examples of RFC

RFC 821 - Simple Mail Transfer Protocol

RFC 768 - User Datagram Protocol

RFC 791 - Internet Protocol

RFC 793 - Transmission Control Protocol

RFC 959 - File Transfer Protocol

RFC 1034 - Domain names - concepts and facilities

RFC 1855 - Netiquette Guidelines

RFC 2573 - SNMP Applications
RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1

RFC 821 - SMTP

RFC 821 is proposed by Jonathan B. Postel from University of Southern California in August 1982

The objective of SMTP is to transfer mail reliably

and efficiently 

Preview

                           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

A Typical SMTP Transaction Scenario

This SMTP example shows mail sent by Smith at host UOW, to Jones, Green, and Brown at host GMAIL.  Here we assume that host UOW contacts host GMAIL directly. The mail is accepted for Jones and Brown. Green does not have a mailbox at host GMAIL.

SMTP Model

Text

               +----------+                +----------+
   +------+    |          |                |          |
   | User |<-->|          |      SMTP      |          |
   +------+    |  Sender- |Commands/Replies| Receiver-|
   +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
   | File |<-->|          |    and Mail    |          |<-->| File |
   |System|    |          |                |          |    |System|
   +------+    +----------+                +----------+    +------+


                Sender-SMTP                Receiver-SMTP

         R: 220 UOW.EDU.AU Simple Mail Transfer Service Ready
         S: HELO GMAIL.COM
         R: 250 GMAIL.COM

         S: MAIL FROM:<Smith@UOW.EDU.AU>
         R: 250 OK

         S: RCPT TO:<Jones@GMAIL.COM>
         R: 250 OK

         S: RCPT TO:<Green@GMAIL.COM>
         R: 550 No such user here

         S: RCPT TO:<Brown@GMAIL.COM>
         R: 250 OK

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

         S: QUIT
         R: 221 GMAIL.COM Service closing transmission channel

Thank You!