POP3 is the abbreviation for Post Office Protocol 3. A protocol that provides a simple, standardised way for users to access mailboxes and download messages to their computers. Most people refer to POP with its version number to avoid confusing it with a point of presence.

Users may operate one or more email POP3's, but it is not recommended that more than one user share a single email POP3.

Otherwise when a user logs into the system and downloads their mail they would be downloading all the emails sent to that POP3, including the email intended for other users.

When you check your email, your e-mail client connects to the POP3 server using port 110. The POP3 server requires an account name and a password. Once you have logged in, the POP3 server opens your text file and allows you to access it. Like the SMTP server, the POP3 server understands a very simple set of text commands. Here are the most common commands:

USER - enter your user ID
PASS - enter your password
QUIT - quit the POP3 server
LIST - list the messages and their size
RETR - retrieve a message, pass it a message number
DELE - delete a message, pass it a message number
TOP - show the top x lines of a message, pass it a message number and the number of lines
Your email client connects to the POP3 server and issues a series of commands to bring copies of your e-mail messages to your local machine. Generally, it will then delete the messages from the server (unless you've told the email client not to).