Fire up your telnet program and tell it to connect to port 110 of pop3.demon.co.uk. From now on you'll be typing commands into the POP3 server. Command can be entered in upper or lower case. All error messages start with '-ERR'. This makes is easy for software to figure-out whether a command has worked.
Humans can read the whole thing and figure-out what went wrong.
From this point on, if you want to disconnect from the server, type 'QUIT' and press enter. If you've marked anything as deleted and then QUIT, not even Demon's staff can retrieve your message. The application should close the connection for you. The next step is to identify yourself to the POP3 server. The normal way to do this is to type the command USER, then press the space key, then type your node name, then press enter.