Simple Mail Transfer Protocol. SMTP is the de facto standard for email transmission across the internet.
SMTP is a relatively simple, text-based protocol, where one or more recepients 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.
SMTP started becoming widely used in the early 1980s. At the time, it was a compliment to UUCP which was better suited to handle e-mail transfers between machines that were intermittantly connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time.
Sendmail was one of the first (if not the first) mail transfer agent to implement SMTP. As of 2001 there are at least 50 programs that implement SMTP as a client (sender of messages) or a server (receiver of messages).
RFC2821 - The Simple Mail Transfer Protocol, which recently obsoleted RFC821
RFC1869 - Defines the capability for SMTP service extensions, creating Extended SMTP, or ESMTP
RFC1891 - Delivery Status Notification (DSN) extension to SMTP