IMAP is an alternative to POP3 (Post Office Protocol 3), works in some fundamentally different ways, and makes a few fundamentally different assumptions.
I’ll examine IMAP, how it compares to POP3, and when you might want to consider using it.
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IMAP: Your messages stay on the server
The biggest single practical difference between IMAP and the older POP3 is that with IMAP, your email is always left on your email service provider’s server.
If your email provider has a limit to how much email can be kept, and you get a lot of email, then IMAP may not be for you.
To put it into perspective: as you might imagine, I get a fair amount of email. The last four years’ worth of email in my personal account uses a little over 40% of the 25 gigabytes of storage provided by Gmail.
Other providers – most notably ISPs – may not be so generous; but then, you may not get multiple gigabytes of email a year like I do. It’s just something to check on.
IMAP: a window into your mail
IMAP creates a master copy of your email stored on the email server. The software you use is just a window: a way of looking at that master repository.
So, when you set up an email program like Thunderbird (or Outlook, or your-favorite-email-program) to access your email via IMAP, or connect your phone or mobile device to your email (which typically also use IMAP) the best way to think of what’s really happening is that the program is simply showing you what’s on the server.
And that’s it.
At least, conceptually.
IMAP: but it does download
If it’s only a view of your email that’s kept on the server, why use a desktop email program at all? You can get a view using a web-based interface just as easily.
I did say the windows analogy is conceptually the best way to think of it. As with many things on your computer, the reality is significantly more complex.
For example, your email program may very well actually download a copy of all newly arrived email to your PC. I think of that as an optimization. You’re still looking at your email as it lives on the mail server’s repository, but your email program has optimized the experience by downloading the email so it can be accessed and displayed more quickly.
In fact, email downloaded by IMAP can be examined off-line, if your email program is appropriately configured. And that’s more-or-less just the same as POP3.
But there is one important difference.
IMAP copies; POP3 moves
When IMAP downloads your email, it is copying the email onto your computer. The original email remains in the email server’s master repository of your email; there’s simply a copy of it on your PC for quick and easy access. (Or backup, as we’ll see in a moment.)
When POP3 downloads your email, on the other hand, it moves the email from the email server to your PC. By default, when a download is complete, the email resides only on the PC to which it was downloaded.
This “copy, not move” difference between IMAP and POP3 enable a couple of very interesting things.
IMAP: Use it on as many devices as you like
Since using IMAP is really only a “view” of the master copy of your email stored on a server … you can certainly have more than one computer open up a view.
In fact, if you’ve got a mobile device accessing your email, you might be using IMAP already, as it’s a very common default configuration for mobile email programs.
Each program using IMAP to access your email is simply keeping itself in sync with the master copy. So if something happens to the master copy – say an email is deleted, or marked as “read” – then that change will be reflected in all the email programs.
Delete a message here, it’s deleted there. Mark it read there, it’ll show up as read here: cross-device synchronization.
IMAP: did I mention folders?
Unlike POP3, IMAP supports folders.
What that means is that if you create a folder on one machine connected to your email account using IMAP, then that folder becomes visible in all email programs connected to that email account via IMAP.
And, of course, if you move a message into a folder, that message is moved into the folder in all email programs connected to that account.
The only common point of confusion is Gmail. Gmail actually doesn’t support folders at all, but instead provides roughly equivalent functionality through the use of labels. Check out my article How do Gmail labels relate to folders? for more.
IMAP: you can upload
In my opinion, this is an under-appreciated feature of IMAP.
If you place a message in your inbox on a machine that is connected to your email account via IMAP, that message is uploaded and placed in the master copy on the server.
In fact, that’s true for any folder, but the inbox has special significance, I think.
It’s what most people want to move when they’re changing email providers.
Moving from Yahoo! to Gmail? Set up a PC-based email program with an IMAP connection to each, and simply drag and drop the contents of the old inbox to the new.
Conceptually, it really is nearly that simple.
Using IMAP for backup
Let’s say you access Gmail via the web and only via the web. You have everything you need on any computer you happen to use, simply by logging into your Gmail account.
What about backup?
A machine running a desktop email program connected to your email account via IMAP makes for a great solution.
In fact, that’s exactly what I recommend these days. Most of your email access may be via your email provider’s web interface, but a machine running an email client like Thunderbird, connected to your accounts via IMAP, will download email as it appears.
As a backup.
IMAP: best when fast and connected; POP3: best for slow or intermittent
The POP3 email protocol was developed in the days of dial-up modems and periodic connections.
Connecting to the internet, downloading all your email, and disconnecting was a common way of life, particularly when no one could use the phone while you were connected.
IMAP leverages a faster and more continuous connection to the internet. It’s more or less constantly checking for updates and synchronization needs between your PC and the master email repository.
Both will work in either scenario. POP3 works just fine if you’re always connected on a fast connection, and IMAP works if connectivity is not always be present and synchronization actions need to be deferred until it is.
But if you are always connected and you are on something faster than a dial-up modem, IMAP might well make for a convenient approach to managing your desktop email.
Assuming that your email provider supports it, of course, and gives you enough space.