IMAP vs POP3 And Which One You Already Use

POP3, the Post Office Protocol, brings mail down to one machine. IMAP, the Internet Message Access Protocol, leaves it on the server so multiple devices all look at the same copy, which is why phones and laptops stay in step. That single difference decides where your mail lives, what your phone can see, which file sits on your disk and what happens the day something breaks.

Most people never chose. A setup wizard chose for them years ago. The consequences turn up much later, usually when a laptop dies or a provider closes an account. Knowing which one you have takes a minute and explains a great deal.

IMAP And POP3, Two Answers to One Question

Both were built to answer one question. How does a program on your desk get at mail sitting on a server somewhere else. They answered it opposite ways.

The names give the design away. Post Office Protocol describes collecting from a pigeonhole and taking it home. Internet Message Access Protocol describes reaching across and reading something that stays put. Neither name has changed. Neither has the behaviour behind it.

What IMAP And POP3 Change on Your Disk

Almost nobody connects the protocol to the file. They are the same decision.

The protocol you picked decides the file your mail client writes. Outlook produces a PST for a POP3 account, because the mail has nowhere else to be. It produces an OST for an IMAP account, because the file is only a local copy of something the server still holds.

Apple Mail writes the answer into its folder names. The suffix follows the account type, so it tells you whether the mail on that disk exists anywhere else. That is worth knowing before somebody wipes the machine.

Where IMAP And POP3 Each Fail

Both work well until they do not. They fail at opposite ends.

laptop stolen
disk fails
Windows reinstalled
somebody deletes
  the wrong folder

mail is gone

account closed
employer revokes it
provider shuts down
password lost with
  no recovery

mail is gone

IMAP is not a backup. The belief that it is causes real losses. Every device copies whatever the server holds, so a deletion on the server reaches every device in seconds. That is synchronisation working correctly. A backup is a copy that nothing can reach and change, which is a separate thing you have to arrange.

How to Tell Which Protocol Your Account Uses

1. Open the account settings in your mail program. The account type is written next to the name in every client, usually as POP3, IMAP or Exchange.
2. Delete a message on your phone and look at the laptop. Gone from both means IMAP. Still there on one means POP3, which also means the two have been drifting apart for as long as you have had them.
3. Look at the file on disk. A PST or a folder ending in popmbox says POP3. An OST or imapmbox says IMAP.
4. Compare the browser with the laptop. Open webmail and count. Far fewer messages there than on your machine means a POP3 account that has been taking mail off the server for years.

That last check finds the situation people most want to know about. Years of mail on one machine, none of it on the server. Nobody notices until that machine is being replaced.

When POP3 Is Still the Right Answer

IMAP is the default for good reasons and it is not always what you want.

Every row is somebody choosing POP3 on purpose for what it does. That is a different thing from having it because a wizard picked it in 2009.

Moving From POP3 to IMAP

Switching is ordinary and the order is the whole job. Doing it in the wrong sequence is how people lose mail while trying to protect it.

1. Add the IMAP account without removing anything. Both accounts sit in the client at once. Nothing is deleted at this stage and nothing needs to be.
2. Copy the mail across inside the client. Drag folders from the old account to the new one and let it upload. This can take hours on a large mailbox and it is the step that must finish.
3. Check the server, not the client. Sign in to webmail and confirm the folders are actually there. The client showing them proves nothing, because the client is also where the old copy lives.
4. Leave the old account in place for a month. Nothing is gained by removing it and the cost of being wrong is everything.

Going the other way needs the same care in reverse. Pulling an IMAP mailbox down with POP3 before an account closes is a reasonable thing to do. The download has to finish before the account does. Providers rarely give notice measured in weeks.

The POP3 Setup That Causes the Most Confusion

One arrangement produces more puzzled questions than the rest put together. POP3, with leave a copy on the server switched on.

Mail downloads to the machine and also stays where it was. Nothing appears wrong. The mailbox fills quietly for years until the provider refuses new mail. By then the person is certain they deleted all of it, because they did, on their laptop.

Deleting locally under POP3 deletes nothing on the server. The two are not connected in that direction and never were. Emptying the server means signing in to webmail and doing it there, which is the step nobody thinks of because the mail already looks gone.

Where an old POP3 machine turns out to hold the only copy of something, reading its file matters more than the protocol argument. Our PST viewer and MBOX viewer open those in a browser tab. The converters move the contents somewhere current.