SMTP - Simple Mail Transfer Protocol
Simple Mail Transfer Protocol is the foremost TCP/IP layer protocol used to send or receive email messages. SMTP by default uses port 25. The functionality of SMTP is limited to "sending or receiving", while the actual retrieval of the message is done through either of the two following protocols: POP or IMAP.
Working of SMTP
SMTP protocol at client system sends the message to the mail server. The message is then retrieved by either POP of IMAP and read through any email client on the receiver's computer. In other words, SMTP is just used to transfer the message to the right inbox.