Simple Mail Transfer Protocol (SMTP)
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Simple Mail Transfer Protocol (SMTP) is an Internet standard for E-mail transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 10), and last updated by RFC 5321 (2008), which describes the protocol in widespread use today, also known as extended SMTP (ESMTP).
While electronic mail server software uses SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) to access their mail box accounts on a mail server.