IMAP4 vs. POP3

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PhoneBoy Explains: IMAP4 vs. POP3 (or: Why You Want IMAP4)

Back in the old days, before dialup Internet access was common, people read their email with Unix-based commands like 'mail.' Once personal computers were able to access networks with TCP/IP, people wanted to be able to read and write email from the comfort of their own PC and not have to use cryptic Unix commands to do so. So a bunch of people got together and hashed out a standard called "Post Office Protocol." While it does a few other things, it's basically designed to download mail from a server (usually Unix-based) onto a personal computer so that a person can use a more intuitive program to read the mail. Most every email program on a personal computer today supports POP3 (or Post Office Protocol, version 3).

If you primarily use one computer to read and write your email from, POP3 works really nice. All your email is stored on your local computer, you can access it and store it as you like. But POP3 works really bad when you use multple sets of computers to read and write email. While I access my email from just about everywhere, there are two primary places I read it: at work and at home. If I were to use POP3 to read my mail, it would "download" all my email to whatever computer I was on at the time. So if I read my email at work and downloaded it all to my computer at work, I would not be able to access that email from home uness I uploaded it back to the server. And if I save a bunch of email at home and want to get at it from work, I can't get at it unless I resend it to myself.

For the very reason of staying in sync, I basically avoid using POP mail clients. To insure I knew where all my email was, I used a Unix shell account and, as necessary, used POP mail clients to download and decode email attachments. But now, thanks to IMAP, I don't have to do that anymore. IMAP stands for "Internet Mail Access Protocol" and is an up and coming standard for mail access. It's not quite as ubiqutous as POP3 is yet, but the current IMAP4 standard is being implemented in many new email products like Netscape Communicator and Microsoft Outlook.

IMAP4 and POP3 are both standards hashed out by the Internet Engineering Task Force, both have their own RFC's (or Request for Comments, which are basically well documented and open Internet-based standards), and they accomplish similar things and work much the same way. IMAP and POP both require a "server" set up that has access to your email. A piece of client software is then configured to access this server. The client and server then "talk," exchanging email as appropriate. These servers are very different, however. You must have a client that supports the protocol and it must be configured correctly. Your ISP should be able to tell you if they support these standards and how you would need to configure your client software to take advantage of it.

IMAP works really well when you use multiple computers to read your email, but even if you only read your email from a single computer, there are two convincing reasons to use IMAP over POP: It will save you time and it's somewhat more secure.

Because POP3 was designed primarily as an "offline" mail reading solution, POP3 clients usually require you to "download" all your email before you read it. IMAP can function this way, but it also works in an "online" and a "disconnected" state. In a disconnected state, you can work with "local" copies of stuff you have downloaded previously. In an online state (usually the default), you access your email directly on the server. You get a listing of messages in each "folder" and download messages only as they are read. You can store your email on the server in "folders" for later retrieval and can easily get at them again. This bandwidth-friendly approach means you only download what you need to and nothing more.

IMAP has an important security feature built into it, which alone makes IMAP worth considering. With POP, your "password" is transmitted in the clear, which means anyone listening on the network can make a note of your password, pretend to be you, and read your mail without you knowing. IMAP implements a sort of "challenge-response" system based on your private password. Your real password is never typed in the clear and the challenge-response is done in such a way that, given the challenge and the response, there is no way to determine what your actual password is. An extension to POP, called Authenticated POP (or APOP), does something similar, but is supported by even fewer email clients than IMAP4 currently is.

Given that not a lot of ISPs are implementing IMAP as of yet, you may not have much choice in the matter as to which standard you can use today. IMAP is still gaining acceptance throughout the Internet community, but when you have the opportunity to use it, you should. The added functionality is quite cool. More information on IMAP can be found on the IMAP server (, maintained at the University of Washington. This site includes a more in-depth comparison of POP3 and IMAP, relevant RFC's, and a listing of clients and servers that currently support IMAP.

Last Update: 1 September 1997

IMAP vs. POP3?

What is IMAP?
Internet Message Access Protocol (IMAP) is a standard protocol for accessing e-mail from your local computer. IMAP is a client/server protocol in which e-mail is received and held for you on the servers. You (by using your e-mail client, such as Outlook Express, Netscape Messenger or Eudora) can view just the header information, which includes the sender and subject of the letter, allowing you to decide whether to download the message or not. You can also create and manipulate folders or mailboxes on the server, delete messages, or search for certain parts or an entire note.

What is POP3?
Post Office Protocol 3 (POP3) is also a standard protocol for receiving e-mail and is also a client/server protocol in which e-mail is received and held for you on the servers. With POP3, your mail is saved in a mailbox on the remote server until you check your mail. When you check your mail, all of the mail is immediately downloaded to your computer and is no longer maintained on the server.

What’s the Difference?
IMAP can be thought of as a remote file server, while POP3 can be thought of as a "store-and-forward" service. Ultimately they both accomplish similar tasks but often one will suit your needs better than the other.

What are the Benefits of IMAP?
Since you can view just the header information without downloading the entire message, you can delete large messages without wasting time for downloading them. Also, because the messages remain on the server, you can access your mail from multiple locations and ensure that your messages are always available for you. And, with the messages remaining on the servers if your computer crashes you don’t have to worry about losing your messages.

What are the Benefits of POP3?
Since all of your messages are downloaded immediately, after you check your mail at your computer, you do not need to actually be connected to the Internet to read your email. Also, because the messages are downloaded to your computer you do not need to worry about accruing disk usage charges because the messages do not stay on the servers.

What are the Drawbacks to IMAP?
Unlike POP3, IMAP requires continual access to the server during the time that you are working with your mail. Also, because the messages are stored on the server, it becomes much easier to go over your disk quota. However, you can reduce this risk by taking specific steps to not leave your Sent Mail, Drafts or Trash on the server. And like any new service, there is a slight learning curve to get used to IMAP since most people are used to using POP3.

What are the Drawbacks to POP3?
The major drawback to POP3 is that it is an older protocol that was designed before people were able to easily send large emails with attachments. Because POP3 downloads all the mail on the server at once, people are occasionally unable to successfully receive their messages because POP3 will get stuck or disconnect when trying to download large messages. To fix this people will either need to contact tech support or log onto our web mail system to delete the large messages themselves. Also, if you use POP3 and are traveling or check your mail from multiple locations, you will not be able to view any of your old mail because the messages only exist on the computer on which you originally received your mail.

(Last Update : January 14, 2003)

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