IMAP (Internet Message Access Protocol) is an Internet based protocol for the Internet. In basic terms it is just another method of retrieving your e-mails. For example most people have either a web-based e-mail address from Yahoo, G-mail, Hotmail or their ISP provider. To get to your messages you do one of the following:
1) You log onto the website for your e-mail and type in your username and password and check your emails.
2) You use Outlook or Thunderbird or some other E-mail application that lives on your computer to check your emails.
Option 2 is what IMAP will drastically effect. When you check your email with any E-mail application that is not web-based, it basically logs unto the mail servers discretely and gets all your E-mails and downloads them to your computer. In most cases the server uses an Internet based web protocol known as POP3(Post Office Protocol). So IMAP and POP3 pretty much do the same job. It is way of getting the E-mails from the mail-servers to our computers. Both do their job well but were created for different uses.
POP3 was mainly meant for people that checked their e-mail on one computer and for offline use. This way they could download all their new e-mail to their computer and now the e-mail only lives on their personal computer. The mail server no longer has a copy of those e-mails, unless one specifies on the POP3 access options to leave messages on the mail-server. POP3 works great for this; the problem comes when we want to check our e-mails in multiple computers and on hand held devices.
If the e-mails are deleted at the mail-server after downloading them to our computer, we will not be able to get those e-mails on another computer or hand held device. But if we choose to keep the e-mails on the mail-server then we can still get those e-mails but there is no synchronizing of information from our computer to the mail-server. For example I have 5 new e-mail messages and I download them to my computer. Say I view all of them and delete 3. When I go to work and check my e-mails, I will get those 5 e-mails as “new” and unread. The ones I deleted will still be there, why? Because I deleted them from my computer, not the mail-server. Additionally, if you put those 3 kept messages into a folder in your personal computer, in the office you will not be able to see those changes. So that is where IMAP comes in.
IMAP is robust enough to handle folders, deleted and already viewed e-mails. In other words if set up properly, no matter what computer you check your e-mail with, an E-mail application does everything and will always be synchronized from one computer to the next. IMAP does this by not downloading your messages and storing them on your computer. Instead, they live on the mail-server; that way any changes you make happen at the mail-server as well. Of course, you can always change things a bit and download the messages locally to your computer, but even then things will stay synchronized.
If you have a Google account and would like to set IMAP here is there help page.
Google’s IMAP support page