The TCP/IP Guide  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 Overview, History and Standards 1 2 3 SMTP Connection and Session Establishment and Termination

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 clear.

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 Providers.

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.

SMTP Overview, History and Standards 1 2 3 SMTP Connection and Session Establishment and Termination