Answer:
The following are the explanation for each command, with the SMTP protocol considering only RFC 821.
Explanation:
The key commands of the SMTP protocols which take the RFC 821 into account are as follows:
HELLO -  
A client receives that order to describe himself  
HELP-  
It asks by the receivers to submit the command information as argument  
QUIT -  
It is used to terminate the message.  
-FROM MAIL  
The clients use it to acknowledge the message's sender.  
NOOP -  
The client does not use any procedure to verify the receiver status as required by the receiver's reply form.  
RCPT-  
The user needs it to determine the intended beneficiary.  
SAML-  
It sends or ends the specific mail which is sent to the mailbox or the SAML
RESET-  
The existing email transaction is aborted.  
SEND- The mail to be sent by the recipient's terminal, not its mailbox, is specified.  
TURN-  
It allows the sender to change position and recipients.  
VRFY-  
Verification checks the recipient's address.