POP3 stands for Post Office Protocol Version 3. POP servers function like post-office boxes in that they hold your mail, or email, until you are ready to retrieve it and read it. In the jargon of the official standards, the computer you sit at and with which you get your email is called a "client host." The computer that stores the email until you ask for it with the POP service is the "server host."
How It Works
The acts of sending and receiving email are two entirely different processes that use entirely different servers. To send email, you upload your message to an SMTP server. That stands for Simple Mail Transfer Protocol.
The SMTP server collects your message and sends it to the Internet and to its destination. Upon arriving at the destination, the message is collected and held by a POP server (or an IMAP server, but more about that later).
When you check your email on a POP server, your computer asks the server to download all the messages addressed to you. You get your email in your email program and the POP server erases those messages from itself.
With POP servers, you control your mail locally with the computer you are using.
Each service provider usually operates its own mail servers. Some providers may not maintain POP3 servers at all.
Among those that do:
Yahoo's is pop.mail.yahoo.com.
Hotmail's is pop3.live.com.
MSN's is pop3.email.msn.com.
See a pattern? If in doubt, ask your email provider's tech support.
RFC1939 is the official document that describes POP3. It describes in technical detail how a POP3 server must function. It was finalized in May 1996.
The original document that standardized POP, RFC918, was written by J. K. Reynolds in October 1984.
POP3 built upon Reynolds' work. Marshall Rose took the first steps, but he shares credit for the POP3 standard with John Myers in RFC1939.
IMAP, or Internet Message Access Protocol, is another way for a server to store your email until you ask to see it.
It's more modern, more flexible and more complicated than POP procedure. You get your email from an IMAP server just like you do from a POP server, but the IMAP server continues to store a copy. The advantage of that is that you can access your email from anywhere, with anything, and you'll always see the same messages. With POP, once you download your mail on one computer, it can no longer be read somewhere else.