SMTP servers commonly use the Transmission Control Protocol on port number 25 (for plaintext) and 587 (for encrypted communications). ==History== === Predecessors to SMTP === Various forms of one-to-one electronic messaging were used in the 1960s.
SMTP grew out of these standards developed during the 1970s. SMTP traces its roots to two implementations described in 1971: the Mail Box Protocol, whose implementation has been disputed, but is discussed in and other RFCs, and the SNDMSG program, which, according to , Ray Tomlinson of BBN invented for TENEX computers to send mail messages across the ARPANET.
Development work continued throughout the 1970s, until the ARPANET transitioned into the modern Internet around 1980. === Original SMTP === In 1980, Jon Postel published which proposed the Mail Transfer Protocol as a replacement of the use of the File Transfer Protocol (FTP) for mail.
SMTP grew out of these standards developed during the 1970s. SMTP traces its roots to two implementations described in 1971: the Mail Box Protocol, whose implementation has been disputed, but is discussed in and other RFCs, and the SNDMSG program, which, according to , Ray Tomlinson of BBN invented for TENEX computers to send mail messages across the ARPANET.
Fewer than 50 hosts were connected to the ARPANET at this time. Further implementations include FTP Mail and Mail Protocol, both from 1973.
Development work continued throughout the 1970s, until the ARPANET transitioned into the modern Internet around 1980. === Original SMTP === In 1980, Jon Postel published which proposed the Mail Transfer Protocol as a replacement of the use of the File Transfer Protocol (FTP) for mail.
In November 1981, Postel published "Simple Mail Transfer Protocol". The SMTP standard was developed around the same time as Usenet, a one-to-many communication network with some similarities. SMTP became widely used in the early 1980s.
For retrieving messages, IMAP and POP3 are standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync. Since SMTP's introduction in 1981, it has been updated, modified and extended multiple times.
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 November 1981, Postel published "Simple Mail Transfer Protocol". The SMTP standard was developed around the same time as Usenet, a one-to-many communication network with some similarities. SMTP became widely used in the early 1980s.
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. Sendmail, released with 4.1cBSD in 1982, soon after was published in November 1981, 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. Sendmail, released with 4.1cBSD in 1982, soon after was published in November 1981, was one of the first mail transfer agents to implement SMTP.
Because of spam concerns most email providers blocklist open relays, making original SMTP essentially impractical for general use on the Internet. === Modern SMTP === In November 1995, defined Extended Simple Mail Transfer Protocol (ESMTP), which established a general structure for all existing and future extensions which aimed to add-in the features missing from the original SMTP.
The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998, but less than 1% in 2002.
ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions. Message submission () and SMTP-AUTH () were introduced in 1998 and 1999, both describing new trends in email delivery.
ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions. Message submission () and SMTP-AUTH () were introduced in 1998 and 1999, both describing new trends in email delivery.
The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998, but less than 1% in 2002.
This does not cause problems in practice, since virtually all modern mail relays are 8-bit clean. Microsoft Exchange Server 2003 advertises 8BITMIME by default, but relaying to a non-8BITMIME peer results in a bounce.
In 2012, the SMTPUTF8 extension was created to support UTF-8 text, allowing international content and addresses in non-Latin scripts like Cyrillic or Chinese. Many people contributed to the core SMTP specifications, among them Jon Postel, Eric Allman, Dave Crocker, Ned Freed, Randall Gellens, John Klensin, and Keith Moore. ==Mail processing model== Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587.
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