POP3 (Post Office Protocol version 3) is an application-layer Internet standard protocol used by email clients to retrieve email messages from a remote mail server over a TCP/IP connection. It is the third and most widely adopted version of the Post Office Protocol, defined in RFC 1939.
The email client (such as Microsoft Outlook, Mozilla Thunderbird, or Apple Mail) connects to the mail server using POP3, typically over TCP port 110 for unencrypted connections or port 995 for SSL/TLS encrypted connections. The client authenticates with a username and password. The session ends when the client disconnects, and any messages marked for deletion are removed from the server.