RFC 821 (rfc821) - Page 1 of 68
Simple Mail Transfer Protocol
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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
