Woa, dude, I know NOTHING about digital UNIX V4.0E, so I can't submit any ideas about how to use PHP to communicate with your desktop, but I was GOING to just suggest that you keep your mail client open and set it to beep when you have mail. Don't know whether Netscape mail can do this either.
Souldn't your console tell you when you have mail like it does on other UN*X systems?
Sorry to be of no help here.
--ph