Dealing with registered users that have invalid email addresses

[criscokid]

I would like to put registered users who have invalid email addresses in to a special group and display a highlighted message on the forums asking to update their email address. I've got that bit sorted.

This is the part I don't know how to do...

When they've update their email address they automatically get moved back to the registered users category. I'm sure it's do-able as it might get dealt with in a similar way when a newly registered user confirms their account.

I don't want to put registered users with invalid email the Users Awaiting group - I'd like them to be kept seperate.