Post Office Protocol, is the oldest and most common protocol. The email program (eg Outlook, Netscape Messenger, and Eudora) connects to the server using POP protocol and downloads the messages to the user's computer, then deletes the messages from the server.
This protocol is not designed to keep messages from the server, it can be seen as a mailbox that temporarily stores messages until the user receives and deletes them.
The POP protocol is not intended to allow manipulation of messages on the server. The intent is to allow messages to be delivered and then deleted from the mailbox.
The advantage of POP is that the mailbox is always clean, so it may be more useful for small-sized Mailbox. For dial-up access it is recommended to use POP.