Simple Mail Transfer Protocol
As more computers were interconnected, especially in the U.S. Government's ARPANET, standards were developed to permit exchange of messages between different operating systems.
[10] INWG discussed protocols for electronic mail in 1979,[11] which was referenced by Jon Postel in his early work on Internet email.
Though Usenet's newsgroups were still propagated with UUCP between servers,[16] UUCP as a mail transport has virtually disappeared[17] along with the "bang paths" it used as message routing headers.
[19] Over time, as BSD Unix became the most popular operating system on the Internet, Sendmail became the most common MTA (mail transfer agent).
[22] Because of spam concerns most email providers blocklist open relays,[23] making original SMTP essentially impractical for general use on the Internet.
In November 1995, RFC 1869 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.
Standards such as Multipurpose Internet Mail Extensions (MIME) were developed to encode binary files for transfer through SMTP.
Mail transfer agents (MTAs) developed after Sendmail also tended to be implemented 8-bit clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8-bit ASCII-like character encoding) via SMTP.
Mojibake was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only ASCII.
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.
The boundary MTA uses DNS to look up the MX (mail exchanger) record for the recipient's domain (the part of the email address on the right of @).
Based on the target host and other factors, the sending MTA selects a recipient server and connects to it to complete the mail exchange.
A mobile user may often be on a network other than that of their normal ISP, and will then find that sending email fails because the configured SMTP server choice is no longer accessible.
These methods were typically used by corporations and institutions such as universities which provided an SMTP server for outbound mail only for use internally within the organisation.
Where a user is mobile, and may use different ISPs to connect to the internet, this kind of usage restriction is onerous, and altering the configured outbound email SMTP server address is impractical.
This more flexible system is friendly to mobile users and allows them to have a fixed choice of configured outbound SMTP server.
Mail services generally accept email submission from clients on one of: Port 2525 and others may be used by some individual providers, but have never been officially supported.
A typical example of sending a message via SMTP to two mailboxes (alice and theboss) located in the same mail domain (example.com) is reproduced in the following session exchange.
[29] The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes.
The information that the client sends in the HELO and MAIL FROM commands are added (not seen in example code) as additional header fields to the message by the receiving server.
[34] Current support is limited, but there is strong interest in broad adoption of RFC 6531 and the related RFCs in countries like China that have a large user base where Latin (ASCII) is a foreign script.
The main identification feature for ESMTP clients is to open a transmission with the command EHLO (Extended HELLO), rather than HELO (Hello, the original RFC 821 standard).
Each service extension is defined in an approved format in subsequent RFCs and registered with the Internet Assigned Numbers Authority (IANA).
For example, spoofing, in which one sender masquerades as someone else, is still possible with SMTP-AUTH unless the server is configured to limit message from-addresses to addresses this AUTHed user is authorized for.
In this scenario both parties take the invalid or unexpected responses as indication that the other does not properly support STARTTLS, defaulting to traditional plain-text mail transfer.
The project stopped accepting submissions on 29 April 2021, and EFF recommended switching to DANE and MTA-STS for discovering information on peers' TLS support.
Utilising DNSSEC, mail server operators are able to publish a hash of their TLS certificate, thereby mitigating the possibility of unencrypted communications.
[52] A newer 2018 RFC 8461 called "SMTP MTA Strict Transport Security (MTA-STS)" aims to address the problem of active adversaries by defining a protocol for mail servers to declare their ability to use secure channels in specific files on the server and specific DNS TXT records.
One example of this is Internet Mail 2000, but neither it, nor any other has made much headway in the face of the network effect of the huge installed base of classic SMTP.
