IMAP and POP are the two ways a mail client fetches mail from a server, and the choice decides where your mail actually lives. That has consequences for what happens when a device dies, how much disk you use, and whether the same mailbox looks the same on your phone and your laptop.
The difference in one line
IMAP leaves mail on the server and shows you a view of it.
POP downloads mail to the device and, by default, removes it from the server.
Everything else follows from that.
Use IMAP
For almost everyone, and certainly for anyone using more than one device.
Read a message on your phone and it is read on your laptop. File it into a folder and the folder exists everywhere. Delete it once and it is deleted everywhere.
Your mail is also on the server, which means it is included in the server's backups and it survives a lost laptop. That alone settles the question for most business use.
When POP still makes sense
Two cases, and they are narrower than the number of people using it.
One device, and you want the mail off the server: typically to stay within a small quota, or because you deliberately want it held locally.
Genuinely unreliable connectivity, where downloading everything once and working offline is easier than a client repeatedly failing to sync.
Outside those, POP is usually a setting somebody chose years ago and nobody revisited.
The setting that causes most mailbox problems
POP clients almost always offer "leave a copy of messages on the server", and it is frequently switched on with no expiry set.
The effect: every message ever received stays on the server forever, while the user sees a tidy local inbox and believes they have been deleting things.
That is the usual explanation for a mailbox that refills after every cleanup, and for an account whose disk is consumed by mail nobody thought they had. Managing mailbox quotas explains finding it.
If you use POP, set the client to remove messages from the server after a period. A fortnight is a sensible figure. That keeps a safety window without accumulating.
What each costs you
IMAP uses server disk. All of your mail counts against your quota, permanently. On a small plan with years of attachments this becomes the constraint.
POP risks everything on one device. Mail that exists only on a laptop is protected by whatever backs up that laptop, which for most people is nothing.
Neither is free. The question is which cost you would rather pay, and for most people server disk is the cheaper one.
Settings that work
Use the secure ports and the full email address as the username. That last point catches people constantly. The username is [email protected], not you.
IMAP: port 993 with SSL/TLS.
POP: port 995 with SSL/TLS.
Sending, either way: port 465 with SSL, or 587 with TLS. Authentication is required for sending: a client configured to send without it is refused, and the error usually blames the password.
Avoid the unencrypted ports (143, 110, 25). They send your password in the clear, and port 25 is blocked on most home and mobile networks anyway. Configuring email in Outlook sets out entering these.
Switching from POP to IMAP
The move most people should make, and it needs one precaution.
Your existing mail is on the device, not the server. Adding an IMAP account gives you an empty mailbox with only new messages in it, and the old mail is still in the POP account locally.
So: add the IMAP account alongside the existing one rather than replacing it. Then copy or drag the old messages from the local folders into the IMAP folders, which uploads them to the server. Confirm they appear in webmail before removing the POP account.
Check your quota first. Uploading ten years of mail into a 2 GB mailbox fills it, and the upload stops partway with no clear message about why.
Folders behave differently
With IMAP, folders live on the server and appear on every device. With POP, folders are local and exist only where they were made.
That is why a POP user who organises mail carefully on their desktop sees nothing but an inbox on their phone. It is not a fault, and it is the clearest everyday demonstration of the difference.
One IMAP habit worth setting: make sure the client is configured to store Sent and Trash on the server rather than locally. A client saving sent mail locally means those messages are missing from every other device, and from your backups.
Webmail always works
Whichever you use, webmail reads directly from the server.
So webmail showing messages your client does not is normal on POP; the client already downloaded and removed them. And webmail showing nothing while your client has years of mail is the same fact from the other direction.
It is also the fastest way to check whether a problem is the server or the client: if webmail works, mail is arriving and the issue is in the client's configuration. Accessing webmail sets out getting in.
A short recommendation
Use IMAP. Set a mailbox quota so it cannot grow without limit. Archive anything older than a year or two off the server if space becomes tight.
Use POP only when you have a specific reason, and if you do, set an expiry on the server copy instead of leaving it forever.
Whichever you choose, the port and the encryption setting have to match or the client fails with an unhelpful message. How to Diagnose a Mail Client That Will Not Connect explains the pairings.