As a sender of newsletters and advertising, we recommend that you participate in the Certified Senders Alliance (CSA). This allows you to ensure that your emails are evaluated with a low spam probability or are not sorted into the spam folder of the respective 1&1 IONOSmailbox.
This is a positive list project of the Association of the German Internet Economy e.V. (eco) in conjunction with the German Direct Marketing Association (DDV).
This centralized positive list is an interface between bulk mail senders and Internet service providers to ensure that legitimate mail reaches its destination easily.
A further advantage of participation is that successful delivery is regulated for several providers.