POP Post Office Protocol The current champ in Internet email mailbox access standards, but its limitations--basically, you connect to a server and download all your messages, which are then deleted from the server--discourage flexibility. Of course, some clients let you leave all messages on the server, and/or refuse to download messages above a certain size. Still, as messages become longer--with multimedia (such as sound or video) objects and the likes--we'll want some flexibility in what we retrieve and when we retrieve it. That's where IMAP comes in. The current version of POP is POP3.