Back in the days when the Internet was just starting to become a popular pasttime for millions of users, there was only one way to transfer email from server to client: the Post Office Protocol (POP). However, due to the rapid proliferation of devices able to retrieve email from an always-on Internet connection, POP, now in version three or POP3, is becoming as antiquated a concept as token ring.
The primary reason POP is outdated is because of the way it retrieves mail from the server. POP is not a syncing protocol - it's ability to retrieve mail messages is through the pull method. This means that a request for information is originated at the client side and ends up at the server, and the server returns the information requested back to the client. Multiple pull requests in a short amount of time can mimic a push request, however, the two are not the same as explained in this excerpt from the Wikipedia article on Pull Technology:
A push can also be simulated using multiple pulls within a short amount of time. For example, when pulling POP3 email messages from a server, a client can make regular pull requests every few minutes. To the user, the email then appears to be pushed, as emails appear to arrive close to real-time. The tradeoff is this places a heavier load on both the server and network in order to function correctly.
The most detracting thing about POP is that it's a one-way transfer of data. Nothing you do with your messages in your desktop client is transferred back to the server, except for deleted messages (and even then, it's marked for deletion until you disconnect from the session). This means that any email you mark as read, starred/flagged, or assigned to folder is forever locked to one client (the client you first downloaded the email to). POP does not synchronize the state of your inbox after changes are made on a local level. This wasn't a problem in the 90's when people weren't accessing their email on multiple devices, but in with the proliferation of multiple computers and the introduction of tablet computing, mail sync through IMAP makes our digital lives much easier (and with less hair pulling).
With IMAP4, enhanced features are available, such as:
Message state information - synchronization of read, flagged and deleted email transfers between multiple clients, even those simultaneously logged in, and the server.
Server-side searches - helps clients avoid downloading messages to perform a stately search, which in turn, lessens the load on the server.
Partial fetch - clients have the ability to download an entire email message, including any attachments, or keep the attachments on the server for later downloading if the connection speed is sub-par.
Support for multiple mailboxes - most users recognize these in the form of folders used to sort and retain read email for archival purposes.
As you can see, there are some very good advantages to using IMAP over POP. The primary advantage, of course, is the ability to keep the same inbox synchronized between multiple devices. Road warriors rejoice!
IMAP stands for Internet Message Access Protocol, but given the fact that it affords users such a responsive expereience when it comes to their email, the I could very well stand for interactive. Also, when it comes to proprietary business-grade email solutions, it's important to remember that each company brands their own implementation of IMAP into their product. Microsoft does this with their ActiveSync protocol, which acts exactly like the standard IMAP protocol with some additional benefits for mobile clients. Apple incorporates standard IMAP with their OS X Server offering, in addition to their push notification server.
I would ask anyone who is serious about getting their email organized in the new year to consider this very important step in setting themselves up for Inbox Zero success. Even if you're not that serious about organizing your email, if you have multiple devices, it may just help you retain your sanity when dealing with what some consider to be the biggest timewaster of their day.