Is GMX Mail German?

I've been hearing a lot about GMX Mail lately, and it seems to be quite popular in Europe. However, I'm not entirely sure of its origin. Could you clarify for me, is GMX Mail German? I've seen its interface and it has a very European feel, but I've also heard some conflicting reports about its country of origin. It would be greatly appreciated if you could shed some light on this matter for me. I'm particularly interested in understanding the company's background and how it came to be so popular in the email service market. Thank you for your time and assistance in this matter.