I launched a new site a couple of weeks ago and have only just realised that Microsoft have been sending bounce replies to my user registration emails (thanks to the built in bounce system)
It would have been great if I had been notified or made aware of this a bit sooner though, instead of having to manually search for users in non-valid states or check the bounce log.
There's a great notice for admins on the home page when server errors or logged, some kind of real time info about how many users are in any of the non-valid states would be very useful.
This could perhaps be in the member stats box on the ACP home screen. Perhaps colour highlighted to bring the admins attention to it if it requires attention.
You could have
Awaiting email confirmation
Awaiting email confirmation (from edit)
Email invalid (bounced)
Next to each, the number of users which this applies to. And clicking that number would return the list of all relevant users so the admin could investigate and take action as required.