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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 Authorization State: User Authentication Process and Commands 1 2 POP3 Update State: Mailbox Update and Session Termination Process and Commands

Typical POP3 Mail Exchange Sequence

The Transaction state is relatively “unstructured” in that commands do not need to be issued in any particular order to meet the requirements of the standard. However, there is a natural progression to how a mailbox is retrieved, and that means the commands are usually used in the following way:

1. The client issues a STAT command to see the number of messages in the mailbox.
   
   
2. The client issues a LIST command and the server tells it the number of each message to be retrieved.
   
   
3. The client issues a RETR command to get the first message and if successful, marks it for deletion with DELE. The client uses RETR/DELE for each successive message.
   

   Figure 308: Post Office Protocol (POP3) Mail Exchange Process

   This diagram shows the typical exchange of commands and replies employed by a POP3 client to retrieve e-mail from a POP3 server. The STAT command is used to get mailbox statistics, followed by the LIST command to obtain a list of message numbers. Each message in turn is then retrieved using RETR and marked for deletion by DELE. (Messages are not actually deleted until the Update state is entered.)

   
   

Table 258 and Figure 308 show a sample access sequence for a mailbox containing two messages that are a total of 574 bytes; the client's commands are highlighted and the server's responses are in italics.

The exact message sent in reply to each command is server-dependent; some just say “+OK” while others provide more descriptive text as I have done here for the responses to the DELE command.

In some cases, a POP3 client may be configured to not delete messages after retrieving them. This is useful, for example, when Web-based access is being combined with a conventional e-mail client program.

POP3 Authorization State: User Authentication Process and Commands 1 2 POP3 Update State: Mailbox Update and Session Termination Process and Commands