Telnet to SMTP Port Common Syntax

When you send mail using the SMTP protocol, you basically have only the
following commands:

HELO
Greet the mail server. Used once per session - at the beginning of the session

MAIL FROM:
Announce who the sender is. Used once per mail, before specifying any recipients for each mail, or after a
RSET

RCPT TO:
Announce who the mail is to. Multiple recipients are allowed, each must have its own RCPT TO: entered immediately after a MAIL FROM:

DATA
Starts mail entry mode. Everything entered on the line following DATA is treated as the body of the message and is sent to the recipients. The DATA  terminates with a . (period) on a line by itself. A mail may be queued or sent immediately when the . is entered. It cannot however be reset at this stage.

RSET
Reset the state of the current transaction. The MAIL FROM: and RCPT TO: for the current transaction are cleared.

QUIT
End the session. No commits happen here.