postfix mail bounces back with 5.0.0
Solution 1:
The destination mail server (for mail.de) has chosen not to disclose the reason it rejected the mail. That reason should normally appear in the Diagnostic-Code line, but here they have replaced it with garbage.
You can guess that they reject mail from freedns hosted domains, which is a reasonable thing to do. But to be 100% sure of the reason for rejection, you will need to contact the postmaster for that domain and hope that they choose to tell you why it was rejected.