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]
August 1982 RFC 821
Simple Mail Transfer Protocol
2. THE SMTP MODEL
The SMTP design is based on the following model of communication: as
the result of a user mail request, the sender-SMTP establishes a
two-way transmission channel to a receiver-SMTP. The receiver-SMTP
may be either the ultimate destination or an intermediate. SMTP
commands are generated by the sender-SMTP and sent to the
receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the
sender-SMTP in response to the commands.
Once the transmission channel is established, the SMTP-sender sends a
MAIL command indicating the sender of the mail. If the SMTP-receiver
can accept mail it responds with an OK reply. The SMTP-sender then
sends a RCPT command identifying a recipient of the mail. If the
SMTP-receiver can accept mail for that recipient it responds with an
OK reply; if not, it responds with a reply rejecting that recipient
(but not the whole mail transaction). The SMTP-sender and
SMTP-receiver may negotiate several recipients. When the recipients
have been negotiated the SMTP-sender sends the mail data, terminating
with a special sequence. If the SMTP-receiver successfully processes
the mail data it responds with an OK reply. The dialog is purposely
lock-step, one-at-a-time.
-------------------------------------------------------------
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
Sender-SMTP Receiver-SMTP
Model for SMTP Use
Figure 1
-------------------------------------------------------------
The SMTP provides mechanisms for the transmission of mail; directly
from the sending user's host to the receiving user's host when the
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RFC 821 August 1982
