SMTP

SMTP (Simple Mail Transfer Protocol) is a TCP/IP
protocol used in sending and receiving e-mail.
However, since it is limited in its ability to queue
messages at the receiving end, it is usually used
with one of two other protocols, POP3 or IMAP,
that let the user save messages in a server
mailbox and download them periodically from the
server. In other words, users typically use a
program that uses SMTP for sending e-mail and
either POP3 or IMAP for receiving e-mail. On Unix-
based systems, sendmail is the most widely-used
SMTP server for e-mail. A commercial package,
Sendmail, includes a POP3 server. Microsoft
Exchange includes an SMTP server and can also
be set up to include POP3 support.
SMTP usually is implemented to operate over
Internet port 25. An alternative to SMTP that is
widely used in Europe is X.400. Many mail servers
now support Extended Simple Mail Transfer
Protocol ( ESMTP), which allows multimedia files to
be delivered as e-mail.