How does GMX email make money?

As a keen observer of the cryptocurrency and finance landscape, I'm intrigued by the workings of GMX email. Could you elaborate on how GMX email generates revenue? Is it through advertising, subscription fees, or perhaps a unique business model? Understanding the financial backbone of such a service is crucial for investors and users alike. Is the revenue stream sustainable, and does it ensure the longevity of the service? I'm eager to learn more about how GMX email makes money and how it plans to grow in the future.