POP

POP is the older design, and hails from an era when intermittent connection via modem (dial-up) was the norm.  POP allows users to retrieve email when connected, and then act on the retrieved messages without needing to stay "on-line."  This is an important benefit when connection charges are expensive.

The basic POP procedure is to retrieve all inbound messages for storage on the client, delete them on server, and then disconnect.  (The email server functions like a mailbox at the Post Office -- a temporary holding area until mail gets to its final destination, your computer.)

Outbound mail is generated on the client, and held for transmission to the email server until the next time the user's connection is active.  After it's uploaded, the server forwards the outgoing mail to other email servers, until it reaches its final destination.

Most POP clients also provide an option to leave copies of email on the server.  In this case, messages are only removed from the server when greater than a certain "age" or when they have been explicitly deleted on the client.  It's the copies on the client that are considered the "real" ones, however, with those left on the server merely temporary backups.