I would like to be able to flag certain users with a request for the user to update their profile. If they change their email address, they would need to re-authorize that address.

I recently sent out a mailing and got several bounces, so I would flag these users to update their profile.

Does this exist yet?

As far as I know, this does not exists. It does not sound too complicaed, so you can probably get someone to code it for you ate www.joomlancers.com

Mikko