The Post Office Protocol version 3 (POP3) as defined in RFC1939 is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. The protocol defines how the server responds to requests sent from an e-mail client.
A POP3 mail server receives e-mails and filters them into the appropriate user folders. The POP3 service makes e-mail messages available for download from a server, enabling a server to host e-mail accounts and provide basic e-mail access. When a user connects to the mail server to retrieve his mail, the messages are downloaded from mail server to the user's hard disk. Virtually all modern e-mail clients and servers support this protocol.
POP3 works over a TCP/IP connection using TCP on network port 110. POP3 listens on this TCP port 110 for connections from e-mail clients, authenticates the client, and manages the connection with the client. The design of POP3 and its procedures supports end-users with intermittent connections (such as dial-up connections), allowing these users to retrieve e-mail when connected and then to view and manipulate the retrieved messages without needing to stay connected. Although most clients have an option to leave mail on server, e-mail clients using POP3 generally connect, retrieve all messages, store them on the user's PC as new messages, delete them from the server, and then disconnect.
