SMTP commands are ASCII messages sent between SMTP hosts.
*Command: Description
*DATA: Begins message composition.
*EXPN <string> : Returns names on the specified mail list.
*HELO <domain> : Returns identity of mail server.
*HELP <command>: Returns information on the specified command.
*MAIL FROM <host> : Initiates a mail session from host.
*NOOP : Causes no action, except acknowledgement from server.
*QUIT : Terminates the mail session.
*RCPT TO <user>: Designates who receives mail.
*RSET : Resets mail connection.
*SAML FROM <host> : Sends mail to user terminal and mailbox.
*SEND FROM <host> : Sends mail to user terminal.
*SOML FROM <host> : Sends mail to user terminal or mailbox.
*TURN : Switches role of receiver and sender.
*VRFY <user> : Verifies the identity of a user.
