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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

TCP/IP Electronic Mail Access and Retrieval Protocols and Methods 1 2 3 TCP/IP Post Office Protocol (POP/POP3)

E-Mail Access and Retrieval Models

For the reasons just examined, there is an advantage to providing more than one way to access a mailbox. RFC 1733, Distributed Electronic Mail Models In IMAP4, describes three different paradigms or models for mail access and retrieval:

• Online Access Model: This is the mode of access that we would all be using in my “ideal world” scenario, where every machine was always connected to the Internet running an SMTP server. You would have constant, direct online access to your mailbox. In the real world, this model is still used by some Internet users, especially those who have UNIX accounts or run their own SMTP servers. I call this direct server access.
  
  
• Offline Access Model: In this paradigm, a user establishes a connection to a server where his or her mailbox is located. The user downloads received messages to the client device, and then deletes them from the server mailbox. All reading and other activity performed on the mail can be done “offline” once the mail has been retrieved.
  
  
• Disconnected Access Model: This is a hybrid of online and offline access. The user downloads messages from the server, so he or she can read or otherwise manipulate them without requiring a continuous connection to the server. However, the mail is not deleted from the server, like in the offline model. At some time in the future, the user connects back with the server and synchronizes any changes made on the local device with the mailbox on the server.
  
  What sort of changes? Examples include marking whether or not a message has been read, to keep track of unread mail, and marking messages to which the user has already replied. These are important tools to help those with busy mailboxes keep track of what they need to do.

TCP/IP Electronic Mail Access and Retrieval Protocols and Methods 1 2 3 TCP/IP Post Office Protocol (POP/POP3)