Simple Mail Transfer Protocol (SMTP) is an Internet term for electronic mail (e-mail) broadcast across Internet Protocol (IP) networks. SMTP was originally distinct in RFC 821 (STD15),[1] as well as last modernized by RFC 5321 (2008)[2] which contains the extensive SMTP (ESMTP) additions, along with is the protocol in prevalent to make use of today. SMTP is specific for outgoing mail transport as well as use port 25.

Though electronic mail servers as well as additional mail transfer representative make use of SMTP to mail as well as receive mail messages, user-level client mail applications normally only utilize SMTP for sending messages to a mail server for relaying. For receiving messages, client applications typically use moreover the Post Office Protocol (POP) or else the Internet Message Access Protocol (IMAP) to access their mail box accounts on a mail server.