Post Office Protocol (POP3) is an application-layer Internet protocol that is used to extract and retrieve email from the mail server. It is the primary protocol behind the mail communication. It works through a supporting email software client. It connects the POP to the remote mail server and downloads the email messages to the recipient’s machine. Most of the email applications use POP protocol and some uses the newer IMAP (Internet Message Access Protocol).
POP uses TCP/IP protocol stack for network connection and it works with SMTP (Simple Mail Transfer Protocol) for email communication in client/server architecture.
There are two versions of POP – POP2 and POP3.
POP2 was an early standard protocol used to receive emails and it requires SMTP to send emails.
POP3 is the latest standard protocol used to send and receive emails and it can be used with or without SMTP.