POP is an acronym standing for Post Office Protocol and is an application-layer Internet standard protocol. Over a TCP/IP connection, POP is used by local email clients to retrieve email messages from a remote server. The protocol has continually evolved and there have been several versions, with the latest most advanced version POP3.
Although made largely obsolete by the IMAP, POP3 allows users to download emails for the server’s inbox directly to their computer. Even when offline, emails are still available.