IMAP/POP3/OUTLOOK Question

Hi, I was wondering if someone could help me explain how Microsoft OUTLOOK knows which emails that it has downloaded from the server, as far as I know, there isn't any way of marking the email in a pop3 queue that they have been either read or downloaded. But then again, I have an inbox with over 20,000 emails in it, and outlook goes through these in less than a few seconds, so, could anyone explain to me how this is done. // Mattias