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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 Internet Message Access Protocol (IMAP/IMAP4)

TCP/IP Internet Message Access Protocol (IMAP/IMAP4) 1 2 3 IMAP General Operation, Client/Server Communication and Session States

IMAP Features

In the mid-1980s, development began at Stanford University on a new protocol that would provide a more capable way of accessing user mailboxes. The result was the Interactive Mail Access Protocol, later renamed the Internet Message Access Protocol (IMAP). IMAP was designed for the specific purpose of providing flexibility in how users access e-mail messages. It in fact can operate in all three of the access modes: online, offline and disconnected access. Of these, it is the online and disconnected access modes that are of interest to most users of the protocol; offline access is similar to how POP works.

IMAP allows a user to do all of the following:

• Access and retrieve mail from a remote server so it can be used locally while retaining it on the server.
  
  
• Set message flags so that the user can keep track of which messages he or she has already seen, already answered, and so on.
  
  
• Manage multiple mailboxes and transfer messages from one mailbox to another. You can organize mail into categories, which is useful for those working on multiple projects, those who are on various mailing lists and so forth.
  
  
• Determine information about a message prior to downloading it, to decide whether or not to retrieve it.
  
  
• Download only portions of a message, such as one body part from a MIME multipart message. This can be quite helpful in cases where large multimedia files are combined with short text elements in a single message.
  
  
• Manage documents other than e-mail. For example, IMAP can be used to access Usenet messages.

Of course, there are some disadvantages to IMAP, but not many. One is that it is more complex, but it's really not that complex, and the protocol has been around for enough years that this is not a big issue. The most important sticking point with IMAP is simply that it is used less commonly than POP, so providers that support it are not as easy to find as those that support POP. This is changing, however, as more people discover IMAP's benefits.

TCP/IP Internet Message Access Protocol (IMAP/IMAP4) 1 2 3 IMAP General Operation, Client/Server Communication and Session States