Both fetch mail. The difference is where the mail lives afterwards, and that difference decides what happens when a laptop is stolen or a phone is replaced.

IMAP - the mailbox is on the server

  • Every device sees the same mailbox: read on the phone, read on the laptop.
  • Folders, drafts and sent mail sync everywhere.
  • The server holds the mail, so the server backup protects it.

POP3 - the mail moves to one device

POP3 downloads and, by default, DELETES from the server. The only copy is then on that device, and it is not in anyone's backup. A phone lost after a POP3 sync takes the mail with it.
  • One device only, in practice.
  • Nothing syncs: read on the laptop is unread on the phone, forever.
  • It works offline and uses no server quota - the two reasons it still exists.

Use IMAP unless you have a specific reason

IMAP  mail.yourdomain.com  993  SSL/TLS\nSMTP  mail.yourdomain.com  465  SSL/TLS

If you are already on POP3

Do not just switch the setting: mail already downloaded is not on the server and will not appear. Add the IMAP account alongside, then copy the old folders into it from the client.

If you must use POP3, turn on "leave a copy on the server" and set it to delete after 30 days. That keeps the quota under control and keeps the mail in the backup.