What does POP stand for in the email protocol POP3?

POP in the email protocol POP3 stands for Post Office Protocol. The “3” identifies the third major version of the protocol, not a word in the acronym.

POP is an application-layer Internet protocol that lets an email client retrieve messages from a mail server. Its original design suited users with temporary or slow connections: a client could download messages, store them locally, and then read them offline. Traditional POP behavior also marked retrieved messages for deletion from the server, although clients can usually be configured to leave copies online.

POP3 is often compared with IMAP, or Internet Message Access Protocol. POP3 emphasizes downloading mail to a device, while IMAP normally keeps messages on the server so multiple devices can view and synchronize the same mailbox. POP3 commonly uses TCP port 110, with encrypted POP3 connections commonly associated with port 995.