Theoretically IMAP can be used like POP; the actual protocol is a functional superset. However, nobody implements it that way. Outlook and Thunderbird treat it as a synchronized viewport on a corporate mail server, so the mail lives on the server and maybe you have a local copy. That doesn't work with ISPs. Nor with Yahoo, which now limits IMAP to seeing only the newest 10,000 messages, even though others can be seen via their webmail.