thing is that it's illegal to maintain a database with "personal details" which ip addresses according to various german courts are (don't ask..
I've actually looked at some of the German decisions, and I didn't see anything that would be a problem for DNSBLs But if you're getting legal advice from the same wizard who told you to do this:
Confidential: Please be advised that the information contained in this email message, including all attached documents or files, is privileged and confidential and is intended only for the use of the individual or individuals addressed. Any other use, dissemination, distribution or copying of this communication is strictly prohibited.
I can understand why your impressions of the law would be so mistaken. R's, John