Hi John,
Sorry if I misunderstood your question. It seems to me that a popup message that comes up within seconds of them clicking on the 'send email' button should prevent them from repeatedly clicking on the button. If you want to show them the notification before they actually send it, that should be pretty straight forward by populating a non-persisted BO that shows the contents of the notification and then when they hit the submit button on that form the Send process is enacted.
You have all the power at your fingertips that you could possible need for this process and resorting to a mailto link shouldn't be necessary. Just decide how you want the system to react when they click on the notification button and then write your rules to provide that behaviour. If you need a hand with it, then just post your desired sequence of events once they click on the button.
Pete