Would be convenient if the notifier could register itself to handle mailto links. There are currently workarounds but most of them involve pasting a short script as the mail handler command. It's less than ideal for 2 reasons:
1) Most of these scripts are too simple to handle more than the simplest type of mailto link ('to' field only). If they are able to handle more than that they usually require downloading a shell script, putting it somewhere, chmoding it, and then pasting the path to it in the mail handler command.
2) My Grandma can't do any of the steps involved in method 1, but she can probably add gmail notifier via the add/remove applications tool in ubuntu (just as an example).