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.  

Response Code  Explanatory Text  
211  (Response to system status or help request).  
214  (Response to help request).  
220  Mail service ready.  
221  Mail service closing connection.  
250  Mail transfer completed.  
251  User not local, forward to <path>.  
354  Start mail message, end with <CRLF><CRLF>.  
421  Mail service unavailable.  
450  Mailbox unavailable.  
451  Local error in processing command.  
452  Insufficient system storage.  
500  Unknown command.  
501  Bad parameter.  
502  Command not implemented.  
503  Bad command sequence.  
504  Parameter not implemented.  
550  Mailbox not found.  
551  User not local, try <path>.  
552  Storage allocation exceeded.  
553  Mailbox name not allowed.  
554  Mail transaction failed.  
