POP3 stands for 'Post Office Protocol'. It's a popular protocol used to connect with an email server. Email client programs such as Outlook Express use POP3 as one of it's main protocol's. POP3 has now been designed so it does not have to relay on SMTP (Simple Mail Transfer Protocol). This standard has now also been built into the Netscape and Microsoft Internet Explorer browsers. Currently it's chief rival is the Internet Message Access Protocol (IMAP).