Senders and Receivers

In RFC 821, the two hosts participating in an SMTP transaction were

described as the "SMTP-sender" and "SMTP-receiver". This document

has been changed to reflect current industry terminology and hence

refers to them as the "SMTP client" (or sometimes just "the client")

and "SMTP server" (or just "the server"), respectively. Since a

given host may act both as server and client in a relay situation,

"receiver" and "sender" terminology is still used where needed for

clarity.

Mail Agents and Message Stores

Additional mail system terminology became common after RFC 821 was

published and, where convenient, is used in this specification. In

particular, SMTP servers and clients provide a mail transport service

and therefore act as "Mail Transfer Agents" (MTAs). "Mail User

Agents" (MUAs or UAs) are normally thought of as the sources and

targets of mail. At the source, an MUA might collect mail to be

transmitted from a user and hand it off to an MTA; the final

("delivery") MTA would be thought of as handing the mail off to an

MUA (or at least transferring responsibility to it, e.g., by

depositing the message in a "message store"). However, while these

terms are used with at least the appearance of great precision in

other environments, the implied boundaries between MUAs and MTAs

often do not accurately match common, and conforming, practices with

Internet mail. Hence, the reader should be cautious about inferring

the strong relationships and responsibilities that might be implied

if these terms were used elsewhere.

Host

For the purposes of this specification, a host is a computer system

attached to the Internet (or, in some cases, to a private TCP/IP

network) and supporting the SMTP protocol. Hosts are known by names

(see the next section); they SHOULD NOT be identified by numerical

addresses, i.e., by address literals as described in Section 4.1.2.