The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages.

<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)

Simple Mail Transfer Protocol

Extended Simple Message Transport Protocol (Reference: SMTP, RFC 1425/1869)

<messaging, protocol> Extended SMTP. Initially defined in RFC 1869 and extended thereafter. See also ETRN. (1997-11-21)

<messaging, protocol> ("Extended TURN") An ESMTP command (first defined in RFC 1985) with which a client asks the server to deliver queued mail to the client via a new ESMTP connection. ETRN supersedes the SMTP "TURN" command in the same way that ESMTP's "EHLO" supersedes SMTP's "HELO". (1997-11-21)

Batched Simple Mail Transfer Protocol (Reference: SMTP)

<networking, standard> The RFC describing SMTP. RFC 2821 supersedes RFC 821. rfc:2821. (2007-06-01)

<messaging, standard> The original RFC defining SMTP. Updated by RFC 2821. rfc:821. (2007-06-01)