POP stands for Post Office Protocol. The current version is 3, thus POP3 is it's name. Mail messages are always delivered via an SMTP server, however once messages have been delivered to a mailbox directory, POP3 takes over. In a nutshell, Mail applications (such as Outlook, Eudora, etc.) communicate with a POP3 server to read e-mail messages that are waiting for the user in a mailbox on the server.