IMAP4 (Internet Mail Access Protocol V4) is a new mail retrieval protocol which can be used as an alternative to the POP3 (Post Office Protocol V3).
The main difference between IMAP4 and POP3 is that with POP3 the messages are only stored temporarily on the mail server until they are downloaded to the email client where the messages are stored, manipulated, searched etc. With IMAP4 the messages are stored permanently on the server, and they are manipulated, searched etc on the mail server.
This difference means that, whichever computer a user uses to access his or her mail, the mail is always accessible when using IMAP4. With POP3, the user would have to copy the email client mailboxes onto the new PC to have access to their old mail. This makes it easier to have 'hot desking' (people just picking a free desk when they arrive at work) with an IMAP4 server like VPOP3 Enterprise.
IMAP4 also makes it possible to have shared message folders because multiple people can access the same IMAP4 account at the same time and read messages at the same time as other people.
VPOP3 Enterprise uses the IMAP4 'ACL' (Access Control List) system to allow users to restrict which people can access which IMAP4 mailboxes and folders. Not many email clients support ACL yet (Mulberry is one which does), so VPOP3 Enterprise's WebMail system allows users to set the access restrictions on IMAP4 folders.
See also: IMAP4 Mail Protocol