Post Office Protocol version 3 (POP3) is a standard mail protocol used to receive emails from a remote server to a local email client. POP3 allows you to download email messages on your local computer and read them even when you are offline.
The protocol has been around for a number of years and is the simplest of protocols used to manage connections to an email client.
POP3 clients typically delete the mail from the server once they’ve downloaded it; therefore it is not a good protocol to use if you receive mail on multiple machines.
Some newer versions of the protocol do offer you the option of leaving the mail on the server.