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What is the SMTP RCPT clause?

Question Background:
What is the SMTP RCPT clause?

Answer:
RCPT is a command used in an electronic mail transaction. Below are the main commands used when mail is sent via SMTP.

HELO - The client "signs on" to the server using the HELO command
MAIL FROM - The client must tell the server who the mail is from
RCPT TO - Who the mail is going to
DATA - Ready for the actual message input
QUIT - If no errors, the message would have been sent, close the connection

So the RCPT clause defines the receiver of the mail message. You cannot specify more than one address at the MAIL FROM clause, however you are able to add many RCPT clauses. You would expect this as mail clients let you send to more than one address per e-mail (CC/BCC). For example:

MAIL FROM: Anyone@anywhere.dom
250 OK - (Server acknowledges and accepts)
RCPT TO: User1@Anotherdomain.dom
250 OK
RCPT TO: User2@Anotherdomain.dom
250 OK
RCPT TO: User3@Anotherdomain.dom
250 OK