POP stands for Post Office Protocol. This is used to describe how e-mail clients interact with mail servers.
The POP3 Server is a type of mail server used for incoming mail.In simple terms, POP servers provide a mail-drop service (a temporary mailbox to leave messages so they can be picked up at the recipient's convenience.)
When users connect to their ISP POP servers, their e-mail software interface with the server and download any messages for them.
POP is only used to receive messages, it is not used to send them.