The Gist of this Article: There are 2 distinct ways (protocols) that your email application can be setup: IMAP and POP. POP is the older protocol. IMAP is newer and better. If you are setup using POP, you may want to change to IMAP. Please see the end of this message for technical definitions of POP (or POP3) and IMAP.

Why is IMAP Better? With IMAP, all your messages stay on the mail server and all changes (read, file, forward, sort, reply, delete) you make to any email on one computer are reflected automatically on any other computer you use to check your email. Also, IMAP configurations do not delete messages from the server automatically as do most POP configurations. If your computer crashes and you don't have a backup, all your email can be easily retrieved if you use IMAP.

Therefore:
• If you access your email account on 2 or more computers (or any device such as a iPhone, Blackberry or other PDA) you should use IMAP.
• If you want to create sub-folders for filed mail and have them accessible from any device, you should use IMAP.
• If two or more people need access to the same email account, they both should use IMAP.
• If you are concerned about backing up your email, you should use IMAP.

The Possible Downside of IMAP: Because IMAP does not delete messages automatically, your mail server "can" get full. You would need to check your email quota, but most mail servers today give you more than adequate space. Don't store that Sponge Bob video you got from your niece in your inbox!!

What Protocol Are You Using Now? Every email account uses one or the other of these protocols. By default, your email client (Outlook, Mac Mail, Thunderbird, etc) will want to setup your account using POP. This means that you are probably using POP now. There are a number of ways to check which protocol you use, but generally, you want to open your account preferences and look for "Account Type."

If you use any kind of “webmail” (Gmail, Yahoo, HotMail, etc), you are using the IMAP protocol. I.e.: if you check your email from a web browser, you are using IMAP.

IMPORTANT!: You cannot "switch" an existing account from POP to IMAP, you have to create a new IMAP account and then delete the old POP account.

FURTER READING:

Some Scenarios and Details: If you access your email via 2 or more devices using POP, you may be frustrated by the fact that any action you take, such as reading, sorting or deleting, doesn't sync with your email account. You may have read and sorted all your new mail on your phone, for example, but when you log back in to email using a browser or application such as Outlook, you're presented with a full inbox of unread messages that you have to re-read and re-organize. What you may not realize is that you have another option that solves these problems: IMAP. It can be a little confusing to learn about different ways to get email on your phone or in an email client such as Mac Mail or Outlook, but this breakdown of the key differences between POP and IMAP should help you decide which way to go.

There are two ways your devices and clients can communicate with email:
 
1. A one-way communication path (POP). Your device asks the mail server for data and pulls it from the servers -- but that's it. Things you do on your device have no effect on the server. If you read a message on your phone, then log in to email, you will see that same message marked as unread. It may start to feel like Groundhog Day.
 
2. A two-way communication path (IMAP). Unlike with POP, your devices talk back to the email servers and sync your changes automatically with IMAP. When you sign in to your email account in a web browser, actions you've taken on your email client or mobile device (like putting a message in a 'work' folder) will also appear in email (your message will already have a 'work' label on it). This all happens automatically once you set up IMAP, so you don't have to read or sort all your mail twice. This is really helpful when accessing email from multiple devices.

DEFINITIONS:

IMAP:
IMAP stands for Internet Mail Access Protocol. The most distinctive feature of IMAP is the fact that your mail messages remain on the server, instead of being downloaded to your computer. This is advantageous if you access your mail from multiple computers, or if you expect to need access to your mail from any computer on the World Wide Web. Checking your mail with a client or web-based environment using this protocol will allow you retain your messages in a universally-accessible place for access whenever and wherever your need them. IMAP supports the use of folders for mail organization, but instead of organizing the messages on your local computer, these folders are kept on the server as well. Another advantage to IMAP is quicker access to mail. If you access your e-mail from a slower connection, such as modem dial-up, you will notice a substantial increase in speed; only the message headers are initially downloaded, so you can choose to download, open, and read only those messages of interest to you at that time. One drawback to using IMAP and saving messages on the server is that you will be restricted by any quota limitation the administrator may set. Once you have stored enough messages to fill your quota, e-mail may be returned to the sender.

POP:
POP or POP3 stands for Post Office Protocol. This mail delivery protocol was the standard for a number of years, particularly when server-side storage space came at a premium. Most distinctively, a client using POP to check your e-mail will retrieve all of the messages from the server and store them on your computer before opening or reading any of them. (You may select an option not to delete these messages from the server with most clients; doing so will cause you to download these messages again each time your check your mail.) This choice is handy when you have an inconveniently small storage quota on your mail server, or you prefer to retain all of your messages locally so that they can be accessed offline. Once downloaded, you can read messages retrieved with POP anytime without reconnecting to the internet. The biggest drawback to this, for many, is that you can only access previously-read messages from the computer onto which you downloaded them. In a multiple-computer environment, this can be a severe disadvantage.