Simple Mail Transfer Protocol

Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821.
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The Transmission Control Protocol/Internet Protocol (TCP/IP) standard protocol that facilitates transfer of electronic-mail messages, specifies how two systems are to interact, and specifies the format of messages used to control the transfer of electronic mail.

<messaging> (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
You should normally start by identifying the local host:
HELO wombat.doc.ic.ac.uk
You can then issue commands to verify an address or expand an alias:
VRFY fred@doc.ic.ac.uk VRFY postmaster
or expand a mailing list:
EXPN c-help
You can even send a message:
MAIL From:fred@doc.ic.ac.uk RCPT To:fred@mailway.doc.ic.ac.uk DATA What is the point? . QUIT
This is useful if you want to find out exactly what is happening to your message at a certain point.
See also Post Office Protocol, RFC 822, sendmail.
(1995-10-17)

SMTP