Even if you make your contact list 99% bounces and 1% real (and every user of the app does the same), I don't see how this becomes a problem for the app's operator. Remove a contact after 1-2 bounces and you're golden.
Fair. Still golden if this needs to be done for all contacts of all users?
If they bounce they are extremely fast to cull.
Who bounces any more?
Everyone except nutters. Google bounces. Microsoft too. I don't know of any provider that doesn't. Surely someone somewhere does not bounce but they are in the minority.