In all client-server based mail systems, mail is received by a mail server. When the mail server receives a piece of mail, it checks to make sure that the intended recipient is a registered user of that mail server. If the intended recipient is not a registered user, the message is returned to its sender. When a user runs the e-mail client software, the software communicates with the mail server allowing the user to read new mail.
Mail is received at the mail server and held until the client application requests the mail.
The mail is then moved to the client's computer and deleted from the mail server. Many recent POP server versions also allow you to leave mail on the server. POP is especially useful when the user is connecting to the server via a dial-up connection to the Internet. This method minimizes the time connected to the server, and thereby reduces connection fees and long-distance phone charges, if any. Since POP downloads messages to the client computer, POP is not recommended for permanent installation on computers used by multiple users.