SMTP is the second part of the POP mail protocol. SMTP (Simple Mail Transfer Protocol) is the part of POP3 email which sends the messages. A simply way to remember what this protocol does is this SMTP (Send Mail To People) and it help you to remember the correct order of the letters. The POP (Post Office Protocol) is the part which receives incoming mail, much like the local post office. Mail comes in (POP) and the mail carriers deliver the mail to the recipients. (SMTP).