SMTP (Simple Mail Transfer Protocol) is a relatively simple, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transferred. It is quite easy to test a SMTP server using the telnet program (see below).
SMTP uses TCP port 25. To determine the SMTP server for a given domain name, the MX (Mail exchange) DNS record is used.
SMTP started becoming widely used in the early 1980s. At the time, it was a complement to UUCP (Unix to Unix CoPy) which was better suited to handle e-mail transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time.
SMTP Security and Spamming
One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined.
The Anti-Spam Research Group of the IRTF is working on a number of E-mail authentication and other proposals for providing simple source authentication that is flexible, lightweight, and scalable.