Post Office Protocol and used with pull e-mail.  This is when your computer fetches e-mail from another mail server.  It can be used by one mail server to fetch e-mail from another e-mail server.  For instance, when you have an on-premise server and your e-mail gets delivered to your provider, and your on-premise mail server periodically collects e-mail from your provider.  (This is ancient technology and was required years back when companies did not have “always-on-internet”.  Although it is outdated, it is still widely used today.)