I've installed an addon to email inactive users and about 10% are returned with failure messages, where the email is now invalid. I need an addon to suspend these accounts until they insert a new email address on their account. Perhaps this could be integrated with email inactive users?
Basically I think a forum is essentially giving false information on user statistics when a proportion of the users are dormant with invalid emails, I'd rather cut those accounts down and remove them from the system.
I'm not sure how much this would cost to develop but it would be handy and I probably would pay for it.