Simple Mail Transfer Protocol, SMTP

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SMTP is the standard protocol for delivering electronic mail over the Internet. Secure SMTP connections are known as SMTPS. Electronic mail servers use the SMTP to send and receive mail messages or e-mail, and in user-level applications the SMTP is used only for sending messages to a server. To receive messages on a user-level application the POP, IMAP, Microsoft Exchange, Lotus Notes or Domino is used to access their mail box.

In the 1960s there were various forms of one-to-one electronic messaging used and the communication was done with specific systems for mainframe computers. As more computers became interconnected, standards were developed for users on different systems to communicate through e-mail. The SMTP standards were developed in the 1970s.

In 1971 fewer than 50 hosts were connected to the US government’s ARPANET. In 1973 FTP Mail and Mail Protocol were implemented for use and development continued until ARPANET was adapted into the modern Internet.

SMTP was introduced in November 1981. It became widely used and similar to the Unix-to-Unix Copy Program (UUCP) which allows the transfer of files, e-mails, and Netnews between computers that are connected intermediately. SMTP works best if the machines are connected to the network constantly. UUCP has all but disappeared and the SMTP is in use today.

Originally SMTP servers were used typically on an in-house network to transfer e-mail. In 1998 and 1999 new forms of e-mail were submitted. Outside e-mail was sent to a main server in the network then distributed to the inside; outgoing mail was sent to the main server then sent to the outside.

With the expanding World Wide Web, SMTP implemented new rules and methods for relaying mail. To alleviate spam, users were authenticated and the rewriting of mail was developed in an attempt to fix problems within the mail; it had positive as well as negative effects. It helped with internal e-mail to correct mistakes, but was dangerous when the mail was generated outside and had to be relayed. By authenticating users, it reduced spam and provided traceability to e-mail abuse. In the process, separating of mail between submissions and relays became a practice and is now the foundation of modern e-mail transfer and e-mail security.

Initially the protocol was text based and did not transfer binary files or characters. Standards were developed to encode these files to transfer through the SMTP. But this created a problem of decoding the differing character set mapping (different languages) between vendors; however the e-mail addresses were not affected. Today the 8BITMIME extension is used which permits binary files to be transmitted as easy as text and the SMTPUTF8 extension allows for international content and addresses.

The typical mail transfer consists of the user submitting an email to a mail server (MSA) using STMP, and then the mail is delivered to a mail transfer agent (MTA), usually a different instance of the same software on the machine. This process can be done on a single machine or split between multiple machines. The SMTP is used when the process occurs between multiple machines. The MTA locates the host, finds the domain name, and connects to the server as an SMTP client. Then the mail is delivered to the mail delivery agent (MDA) and saved in a mailbox format. The final process is the authorized user will access the mailbox through a program like Outlook, Yahoo, or other e-mail providers. The SMTP is the transfer of the message, not the message itself.