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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 Access and Retrieval Protocols and Methods                           9  TCP/IP Post Office Protocol (POP/POP3)

POP3 General Operation, Client/Server Communication and Session States 1 2 3 POP3 Transaction State: Mail and Information Exchange Process and Commands

Standard POP3 Authentication

The normal method of authorization in POP3 is a standard “username / password” login. This is pretty much identical to how a login is performed in FTP; even the commands are the same. First the client issues a USER command along with the user's mailbox name (his or her user name or e-mail address). The server responds with an intermediate acknowledgment. The client then uses the PASS command to send the user's password. Assuming the login is valid, the server responds to the client with an acknowledgment that indicates successful authentication. The response will also typically specify the number of messages waiting for the user in the mailbox. This process is illustrated in Figure 307.

Table 256 shows an example POP3 authorization, with the client's commands highlighted and the server's responses in italics.

If authorization is successful, the POP3 session transitions to the Transaction state where mail access commands can be performed. If the user name or password are incorrect, an error response is given and the session cannot proceed. The authorization may also fail due to technical problems, such as an inability by the server to lock the mailbox (perhaps due to new mail arriving via SMTP).

POP3 General Operation, Client/Server Communication and Session States 1 2 3 POP3 Transaction State: Mail and Information Exchange Process and Commands