4.8.1.2. POP3 states

The protocol begins with the server displaying a greeting message, usually containing information about the server.

After the greeting message the client takes control and the protocol enters the AUTHORIZATION state where the user has to pass credentials proving his/her identity.

After successful authentication the protocol enters TRANSACTION state where mail access commands can be issued.

When the client has finished processing, it issues a QUIT command and the connection is closed.