Thanks kh99, vBulletin seems complicated!
Could you define a 'bad email address' as one that doesn't exist in database A therefore cannot be added to database B?
In answer to your last question, the problem here is that not all email addresses will be from the same domain. A user could have registered on the wordpress site using an email.
Thank you for your reply, I can't figure this out though. Need to think it through again.