RFC 788 (rfc788) - Page 2 of 62
Simple Mail Transfer Protocol
Alternative Format: Original Text Document
November 1981 RFC 788
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
full-duplex 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.
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+----------+ +----------+
+------+ | | | |
| User || | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP || SMTP | +------+
| File || | and Mail | || File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
Sender-SMTP Receiver-SMTP
Model for SMTP Use
Figure 1
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The SMTP provides mechanisms for the transmission of mail; directly
from the sending user's host to the receiving user's host when the
