As a sender of newsletters and advertising we recommend that you participate in the Certified Senders Alliance (CSA).
This is a positive list project by Verband der deutschen Internetwirtschaft e.V. (eco) in cooperation with Deutscher Direktmarketing Verband (DDV).
This centralised positive list provides an interface between mass mail senders and internet service providers with the aim of ensuring that desirable email from trustworthy sources reaches its targets without any trouble.
Another advantage of your participation is that successful delivery is managed for multiple providers.
More information is available here.