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

A session between a POP3 client and a POP3 server begins when the client sends a TCP connection request to the server. The connection is established using the standard TCP “three-way handshake”, and the POP3 session commences. The first of the three states of a POP3 session is the Authorization state, which is responsible for authenticating the POP3 client with the server.

When the session first enters this state, the server sends a greeting message to the client. This tells the client that the connection is alive and ready for the client to send the first command. An example of such a greeting would be:

+OK POP3 server ready

The client is now required to authenticate the user who is trying to access a mailbox. This both proves that the user has the right to access the server, and identifies the user so the server knows which mailbox is being requested.

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