Simple Mail Transfer Protocol

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
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. SMTP uses TCP port 25.

SMTP started becoming widely used in the early 1980s. At the time, it was a
complement to UUCP 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.

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).
Some other popular SMTP server programs include IBM's Postfix, D. J.
Bernstein's Qmail, and Microsoft Exchange.

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. Today, most SMTP servers support the
8BITMIME extension, permitting binary files to be transmitted almost as
easily as plain text.

SMTP does not allow one to retrieve messages from a remote server. To do
this one must use POP3 or IMAP.

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.

Related RFCs
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