Reply To: IMAP vs POP3
No, IMAP and POP are quite distinct protocols. For starters, IMAP requires a tag for every command, many implementations choose consecutive integers (converted to text of course).
The whole idea of IMAP retaining mail while POP does not is an implementation choice for each. If you want, your POP client does not have to issue delete commands for messages received, and an IMAP could equally choose to delete everything it receives and work only from a local cache. The difference is that IMAP explicitly has commands for moving messages around, and if I’m not mistaken, for accepting messages from the client to be stored on the remote mail store. Because of this, it is rare for IMAP clients to automatically delete messages, as it is intended as a centralized repository for the mail for any one account.
If you see email on both/all your devices using POP, it just means it’s likely that none of them is set to delete mail from the post office after retrieval. It may be possible that software syncs between devices in that if some message is deleted from the post office, it gets rid of the local copy too, but none of that is required behavior by the protocol.
IMAP is generally preferred because of these conventions, so that as Christian says, there is a unified view of messages, folders, and generally everything; clients are typically programmed to keep a cache locally of the server contents, and is able to reconcile its cache with anything other IMAP clients do. So for example if you move a message from INBOX to Verizon on your mobile (let’s say it’s an email from Verizon telling you your bill is available for download), the desktop can see this move was made and update its display and cache accordingly. IIRC, the protocol allows constant connection and notifications of events like this.