Government's ARPANET, standards were developed to permit exchange of messages between different operating systems. As more computers were interconnected, especially in the U.S. Users communicated using systems developed for specific mainframe computers. Various forms of one-to-one electronic messaging were used in the 1960s. SMTP servers commonly use the Transmission Control Protocol on port number 25 (for plaintext) and 587 (for encrypted communications). The protocol version in common use today has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalized email addresses. It has been updated, modified and extended multiple times.
SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971.
For retrieving messages, IMAP (which replaced the older POP3) is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. Mail servers and other message transfer agents use SMTP to send and receive mail messages. The Simple Mail Transfer Protocol ( SMTP) is an Internet standard communication protocol for electronic mail transmission.
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