SPAM filtering MUST be implemented as voluntary opt-in, otherwise it will be illegal, as you correctly notice. Spam-filtering can only be legal when it is performed strictly on behalf of the recipient, the receipient is in full control over the behaviour all the time, and the blocking or deletion of any messages is performed with explicit and voluntary (i.e. condition-free) consent of the receipient himself.
If that were true, there would be no Gmail, Hotmail, or Yahoo mail users in Germany, since they offer only limited control over spam filtering, and some stuff, e.g. DNSBL blocks, is unconditional.
I expect that the mail systems make it a condition of subscription that the recipients consent to the mail system doing whatever it has to do to filter incoming mail.
Regards, John Levine, johnl@xxxxxxxxx, Taughannock Networks, Trumansburg NY Please consider the environment before reading this e-mail.