(1) The full name of Pop is Post Office Protocol which is Special to receive letters.

(2) POP (Post Office Protocol) is used by the client to retrieve mail from an Internet server.

(3) Post Office Protocol version 3: a common protocol for transferring mail from the server ('Post Office') to the client. Defined in RFC 1939.

(4) I knew such things existed, and that most of them used a simple application protocol called POP (Post Office Protocol).

(5) For a better understanding, it is recommended to read the official POP3 specification, RFC1939 from IETF: Post Office Protocol - Version 3.