The POP protocol requires an additional server to run on the computer with the mailbox. A user runs e-mail software that becomes a client of the POP server to access the contents of the mailbox. There are several differences between an e-mail server and a POP server. The mail server uses the SMTP protocol, while the POP server uses the POP protocol. The mail server accepts a message from an arbitrary sender, while the POP server only allows a user to access the mailbox after the user enters authentication information.
POP is especially popular among users who rely on dialup telephone connections.
They can attach to a modem and use a telephone line to form a connection either directly to the mailbox computer or to some other computer on the Internet. Once connected to the Internet, the user can run a POP client to contact the server and access e-mail.