there is another way, however its not an "email notifiction"
if you use Chrome, therre's an extension called "Chrome life". That lists which of your friends is online.
You do need to be logged into your "https://my.secondlife.com" link in order for it to work,
but after that, you can leave it open, and minimize it.
https://chrome.google.com/webstore/detail/chromelife/bggofoofijfepafnpkfabkahekhlhohg