Co-registration leads are bad for mailers because users are subscribed by check boxes, which they may or may not of intended to check instead of them going directly to you for specific information they want. These co-registration leads will eventually complain about the multiple lists they are subscribed to, which will in turn hurt inbox delivery.There are are also no records of any subscribe request, with co-registration. You are putting your trust into companies that are selling you email addresses of users who possibly were never interested in your content in the first place.