IMAP (short for Internet Message Access Protocol, and also known as IMAP4 or IMAP4rev1) is the name of a protocol which allows you to view e-mail on a server without having to first download it onto your current computer (or any other device, such as a mobile phone, if supported).
IMAP is not as common a protocol as POP as it requires large amounts of resources for space (all e-mail is stored on the server, and not your locally) and processing (when browsing or searching mailboxes). But, one of it's biggest advantages is that the e-mails are stored on the server: you can connection with all different machines, programs and devices (such as your desktop, laptop and mobile phone) all at the same time and they won't affect each other.
For more details about POP, please see the following article:
So, how does IMAP work?
IMAP works by connecting to the server, logging on using your username and password and then downloading a list of available folders on the server (the subscription list). With IMAP you can create number of different mailboxes under your inbox, and this list tells the program what's available on the account.
When you then click on a mailbox (or if you're first loading the program, getting the e-mails from the inbox), the program will get a list of e-mails in the mailbox. From this list (which doesn't contain any information about the e-mail, just the unique ID of all the e-mails in your account), it can then do a number of things, including:
- Download the headers (these are the informational lines at the top of the e-mail, and describe who sent it, when it was sent, etc.);
- Download the whole e-mail (including the headers);
- Change the status (such as read or not-read);
- Move the e-mail between folders;
- Mark the e-mail for deletion;
- Delete the e-mail.
Most e-mail programs are set to download the whole of the e-mail onto your computer to view, and then mark it read after a number of seconds of reading it.
