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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 Access and Retrieval Protocols and Methods
9 TCP/IP Post Office Protocol (POP/POP3)
POP3 Transaction State: Mail and Information Exchange Process and Commands
(Page 2 of 2)
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:
- The client issues a STAT command
to see the number of messages in the mailbox.
- The client issues a LIST command
and the server tells it the number of each message to be retrieved.
- 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.)
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.
Table 258: Example POP3 Mail Exchange Process
+OK 2 574
(Message 1 is sent)
+OK message 1 deleted
(Message 2 is sent)
+OK message 2 deleted
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.
Key Concept: After successful authorization, the POP3 session transitions to the Transaction state, where the client actually accesses e-mail messages on the server. The client normally begins by first retrieving statistics about the mailbox from the server, and obtaining a list of the messages in the mailbox. The client then retrieving each message one at a time, marking each retrieved message for deletion on the server.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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