In case Az. 7 O 80/07, the District Court of Lüneburg has ruled that the use of blacklists for mail filtering is an illegal process. The court thereby confirmed the view of a known spammer that the fact that mails from his servers were deleted by the SPAM filter was an act of censorship.
According to the ruling, the fact that a mail server is used to transmit soleily SPAM is not sufficient to block mails from it entirely. Blocking a single mail address would have been sufficient, according to the court. However, even this step would only have been acceptable in order to prevent an immanent danger of a virus attack.
From a mail filtering point of view, however, this view is very unsatisfactory. Nowadays, most of the SPAM mails are indeed originating from hosts which are used soleily for spamming purposes (dedicated SPAM servers or hijacked Windows clients in bot networks), but the addresses used contain an unique ID most of the time which is used for only one mail.