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).
Since this protocol started out as purely ASCII text-based, it did not deal well with binary files. Standards such as MIME were developed to encode binary files for transfer through SMTP.
SMTP does not allow one to retrieve messages from a remote server. To do this one must use POP3 or IMAP.
Related RFCs
RFC 2821 - The Simple Mail Transfer Protocol, which recently obsoleted RFC 821
RFC 1869 - Defines the capability for SMTP service extensions, creating Extended SMTP, or ESMTP
RFC 1891 - Delivery Status Notification (DSN) extension to SMTP
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