4. THE SMTP SPECIFICATIONS
4.1. SMTP COMMANDS
4.1.1. COMMAND SEMANTICS
The SMTP commands define the mail transfer or the mail system
function requested by the user. SMTP commands are character
strings terminated by . The command codes themselves are
alphabetic characters terminated by if parameters follow
and otherwise. The syntax of mailboxes must conform to
receiver site conventions. The SMTP commands are discussed
below. The SMTP replies are discussed in the Section 4.2.
A mail transaction involves several data objects which are
communicated as arguments to different commands. The
reverse-path is the argument of the MAIL command, the
forward-path is the argument of the RCPT command, and the mail
data is the argument of the DATA command. These arguments or
data objects must be transmitted and held pending the
confirmation communicated by the end of mail data indication
which finalizes the transaction. The model for this is that
distinct buffers are provided to hold the types of data
objects, that is, there is a reverse-path buffer, a
forward-path buffer, and a mail data buffer. Specific commands
cause information to be appended to a specific buffer, or cause
one or more buffers to be cleared.
HELLO (HELO)
This command is used to identify the sender-SMTP to the
receiver-SMTP. The argument field contains the host name of
the sender-SMTP.
The receiver-SMTP identifies itself to the sender-SMTP in
the connection greeting reply, and in the response to this
command.
This command and an OK reply to it confirm that both the
sender-SMTP and the receiver-SMTP are in the initial state,
that is, there is no transaction in progress and all state
tables and buffers are cleared.
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