Because of spam concerns most email providers blocklist open relays, making original SMTP essentially impractical for general use on the Internet. The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998, but less than 1% in 2002. Due to absence of a proper authentication mechanism, by design every SMTP server was an open mail relay. The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial man-in-the-middle attack, spoofing, and spamming, and requiring any binary data to be encoded to readable text before transmission. Over time, as BSD Unix became the most popular operating system on the Internet, Sendmail became the most common MTA (mail transfer agent). Sendmail, released with 4.1cBSD in 1983, was one of the first mail transfer agents to implement SMTP. Though Usenet's newsgroups were still propagated with UUCP between servers, UUCP as a mail transport has virtually disappeared along with the " bang paths" it used as message routing headers. Both used a store and forward mechanism and are examples of push technology. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time.
At the time, it was a complement to the Unix to Unix Copy Program (UUCP), which was better suited for handling email transfers between machines that were intermittently connected.
SMTP became widely used in the early 1980s. The SMTP standard was developed around the same time as Usenet, a one-to-many communication network with some similarities. In November 1981, Postel published RFC 788 "Simple Mail Transfer Protocol". RFC 780 of May 1981 removed all references to FTP and allocated port 57 for TCP and UDP, an allocation that has since been removed by IANA. In 1980, Jon Postel and Suzanne Sluizer published RFC 772 which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail. Through RFC 561, RFC 680, RFC 724, and finally RFC 733 in November 1977, a standardized framework for "electronic mail" using FTP mail servers on was developed. The use of the File Transfer Protocol (FTP) for "network mail" on the ARPANET was proposed in RFC 469 in March 1973. A further proposal for a Mail Protocol was made in RFC 524 in June 1973, which was not implemented. Mail on the ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented, but is discussed in RFC 196 and the SNDMSG program, which Ray Tomlinson of BBN adapted that year to send messages across two computers on the ARPANET. SMTP grew out of these standards developed during the 1970s. 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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