The Full form of POP is Post Office Protocol. It is used as a protocol to retrieve e-mail from a mail server. Most e-mail applications (sometimes called an e-mail client) use the POP protocol, although some can use the newer IMAP (Internet Message Access Protocol). There are two versions of Post Office Protocol ie. POP2 and POP3. The first, called POP2, became a standard in the mid-80’s and requires SMTP (Simple Mail Transfer Protocol) to send messages. The second one and the newer version, POP3, can be used with or without SMTP.