If the domain is bitbin.de and the mail server is hosted on the same server, it's probably Hetzner being scorched-earth for deliverability, just like any other major hosting provider will be. Email from AWS, Rackspace, Heroku etc. get a drastically higher starting spam score because people fire up servers there to spam.
Running the email through something like Amazon SES or Mandrill would probably be helpful. Both have generous free tiers.
Running the email through something like Amazon SES or Mandrill would probably be helpful. Both have generous free tiers.