SMTP (abbreviated Simple Mail Transfer Protocol) is an Internet standard for the transmission of electronic mail (email) over IP (Internet Protocol). This protocol handles the delivery of mail to the recipient's mailbox, who can then access and retrieve messages at any time using the protocols IMAP or POP3.
SMTP traffic consists of three basic commands/responses:
MAIL is the command that introduces the return address, RCPT is the command that introduces the recipient of the message, and DATA sends the actual text of the message.
While electronic mail servers use SMTP to both receive and send messages, user applications use SMTP only to send messages. To receive them, these applications typically use POP3 or IMAP protocols (or Microsoft Exchange, for example).