The short answer
Use IMAP. It keeps your email on the server and syncs folders, read status and sent mail across every device you use. POP3 downloads messages to one device and, usually, removes them from the server.
How IMAP works
With IMAP your mail server is the master copy. Your phone, laptop and webmail all look at the same mailbox, so a message you read on your phone shows as read on your laptop, and a reply you send from one device appears in Sent everywhere.
How POP3 works
POP3 was designed when people had one computer. It downloads new mail to that computer and often deletes it from the server. Check email on a second device and you either miss messages or end up with duplicates.
When POP3 still makes sense
Very occasionally, a mailbox with a tiny storage allowance or a strict archiving setup may use POP3. For almost every small business, IMAP is simpler and safer.
Switching from POP3 to IMAP
Most providers support both. Add your account again in your email app using the IMAP settings, check everything has synced, then remove the old POP3 account. If old messages exist only on one computer, copy them into an IMAP folder first.
- Find your provider's IMAP settings
- Add the account as IMAP
- Copy any local-only mail into IMAP folders
- Remove the POP3 account