Simple Mail Transfer Protocol
(SMTP) A protocol defined in STD 10, RFC 821, used to transfer electronic mail between computers, usually over Ethernet. It is a server to server protocol, so other protocols are used to access the messages. The SMTP dialog usually happens in the background under the control of the message transfer agent, e.g. sendmail but it is possible to interact with an SMTP server using telnet to connect to the normal SMTP port, 25. E.g.
telnet mhs-relay.ac.uk 25
VRFY email@example.com VRFY postmaster
or expand a mailing list:
You can even send a message:
RCPT To: DATA What is the point? . QUIT