Every time a user registers for my site, they have the option during registration of clicking on some check boxes that signify which newsletters of my company they want to subscribe for. To avoid spam hitting my company email domain, we use Constant Contact which is an email distributor, newletter distributor, email marketing service...that we have used in the past. They have an ASP.NET compliant API to call them and setup users for the newsletter in their system. Ideally, once they are created in Kentico's user registration, I would then send the information to ConstantContact to register them for the newsletters they have chosen (if any). I am using a CustomRegistration form and the Custom Registration web part.