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9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)
SMTP Communication and Message Transport Methods, Client/Server Roles and Terminology
(Page 3 of 3)
SMTP Terminology: Client/Server and Sender/Receiver
The original RFC 821 standard referred
to the device that initiates an SMTP e-mail transfer as the sender
and the one that responds to it as the receiver. These terms
were changed to client and server in RFC 2821 to reflect
current industry terminology. Strictly speaking, this is correct,
but in some ways the more current terminology is significantly less
As I explained in the general
discussion of TCP/IP client/server operation,
the terms client and server are used in many
different sense in networking, and this often leads to confusion. In
common parlance, the computers that handle e-mail on the Internet are
usually all called SMTP servers. This is for two reasons. First,
they run SMTP server software to provide SMTP services to client machines,
such as end-user PCs. Second, these devices are usually dedicated hardware
servers running in network centers, typically managed by Internet Service
However, the terms client
and server are now used to refer to the roles in a particular
SMTP communication as well. Since all SMTP servers both send and receive
e-mail, they all act as both clients and servers transactionally at
different times. An SMTP server that is relaying an e-mail will act
as both for that message, receiving it as a server and then sending
it to the next server as a client.
Adding to this potential confusion
is the fact that the initial stage in sending an e-mail is from the
sender's client machine to the sender's local SMTP server. Thus, the
client role in an SMTP transaction may not be an actual SMTP server,
but the server role will always be a server.
Confused yet? J
Me too. For all of these reasons, the old terms sender and
receiver are still used in places in RFC 2821, where needed
for clarity. I consider them much more straight-forward and use them
in the topics that follow.
Key Concept: SMTP servers both send and receive e-mail; the device sending mail acts as a client for that transaction; the one receiving it acts as a server. To avoid confusion, it is easier to refer to the device sending e-mail as the SMTP sender and the one receiving as the SMTP receiver; these were the terms used when SMTP was originally created.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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