For your rejections to be so host specific like that it'll be one of 2 things:
1/ Your current hosting provider does not trust that domain and will not allow you to send emails to that domain.
2/ The domain (freenet.de) does not trust your hosting provider and rejects emails from your provider.
First thing is to check if you can send from your email client as it may be down to a lack of authentication in website created emails. If your personal email client works you need to try authenticating with your SMTP server to send emails (with another plugin like SMTP Mailer). If this fails speak to your hosting provider and then maybe freenet.de if needed.