Simple Mail Transfer Protocol (SMTP) is one of the core Internet protocols. It is used to route and deliver email messages from sender to recipient domains. SMTP was defined as a standard in 1982 and in 2008 was revised to become Extended SMTP, which is the version widely used now. SMTP is not used by clients mail applications to receive mail. This is done via POP3, IMAP, or other mechanisms such as Active Sync in Microsoft Outlook.