When you retrieve a message using IMAP, your e-mail client makes a local copy, but a copy also remains on the server (until you delete it). Better yet, IMAP saves a lot of information about your message—for example, whether you’ve read, filed, or forwarded it. The result is your inbox looks the same whether you’re looking at it on your iPhone or your Mac. By contrast, when you use POP to retrieve a message, your local copy becomes your only copy. The message is typically deleted from the server. Even if you tell your e-mail client to leave a message on the server after downloading it, the server won’t know whether you’ve read or replied to it.
POP uses bandwidth more efficiently, which is good for people with very slow Internet connections, and it doesn’t impose inbox storage quotas. But IMAP offers a host of other advantages. Here are four big reasons I think you should switch today. (After you’re convinced, learn how to make the switch by reading The IMAP advantage.)
1. Avoid webmail outages
Not long ago, Gmail suffered an outage that affected only users who accessed their e-mail via the Web. Users who connected to their Gmail accounts using IMAP were unaffected, and could continue to retrieve their mail as usual. To be sure, such problems are extremely infrequent, but even so, the moral is that having more than one way to access any given e-mail account can be extremely useful. The vast majority of e-mail providers that offer IMAP access also let you access your mail on the Web, and many (though not all) Webmail providers also let you use IMAP.
2. Switch clients or platforms painlessly
Suppose you decide one day to switch from Microsoft’s Entourage 2008 to Apple’s Mail, or from Qualcomm’s Eudora to Mozilla’s Thunderbird. If you use POP, switching clients can be a huge aggravation. You may have to export messages from your old client, import them into the new client, or both. Either way, in the process, you risk losing messages or the metadata attached to them, because of fundamental differences in the storage systems various clients use. It’s such a hassle that if you have hundreds of thousands of messages, it may discourage you from moving to another program that could serve you better.
With IMAP, though, this sort of problem goes away magically. Switching e-mail clients is as simple as entering your credentials and a couple of settings in a new program. Then you wait while some or all of your messages download from the server. Messages need never be exported or imported at all, and your new client will show the same mailboxes, flags, and message organization as the old one. You can even switch (as I do) among several different clients at any time—for example, if you generally prefer Mail but occasionally want to use features that only Entourage has, or if you want to try out new e-mail software without committing yourself.
3. Read all your mail on multiple devices
If you want to access your e-mail using a Windows or Linux computer, an iPhone, an iPod touch, or some other portable device, simply open an IMAP client on the other system, enter your settings, and you’re off. For people who must use a variety of devices or operating systems, IMAP is a tremendous convenience in that it lets you see exactly the same data—including saved, filed, and sent messages—on every device. Macworld’s Rob Griffiths explores this aspect of IMAP in his video "Manage e-mail on multiple Macs."
4. Keep an extra copy of your messages
Although I always recommend backing up all the data on your Mac (including, of course, your e-mail), the fact that IMAP gives you both local and server-based copies of each message can help you avoid data loss. For example, if your disk crashes or your computer is stolen or damaged, you could lose all your locally stored messages, but with IMAP, the server would still safely hold master copies of all of them. Conversely, if your e-mail provider were to suffer data loss on their mail servers, your local copy of each message would serve as a backup.
Senior Contributor Joe Kissell is the senior editor of TidBits and author of numerous ebooks about OS X.