GLOSSARY: SMTP

The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for sending e-mails. SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971.

Most internet email is moved around the world using SMTP. It defines a standard template of commands and formatting that allow different mail programs, on a huge range of computers, to understand one another. Protocols are used to specify a set of special messages that should be exchanged between computers to achieve a particular functionality, in this case the delivery of email.

SMTP was defined when the internet had only a tiny number of users, so the original specification did not include any way for computers to authenticate one another, i.e. there was no way of knowing if the message claiming to come from TrustedBank actually came from TrustedBank’s computers.

This weakness was addressed in a later extension to SMTP called SMTP-AUTH. This protocol version is in common use today and has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalised email addresses. But it was not 'required', and so almost all mail servers still accept unauthenticated messages.