Spamhaus and other well-known DNSBLs don't yet support IPv6, and I'm not sure they ever will. What alternatives are there for spam filtering on IPv6?
One approach is to rely entirely on content-based filtering which requires that the MTA receive the whole content of the email before deciding whether to accept or reject it - which will significantly increase the inbound bandwidth requirements for a public MTA.
Another possible approach is to require SPF for inbound email and combine it with a blacklist of sending domains (as distinct from sending MTAs, as at present with DNSBLs). This would require all mail-forwarders to rewrite the envelope-sender, which is the underlying reason why SPF doesn't really work. DKIM works fine, but doesn't get the advantage of dropping the connection before DATA is reached in the SMTP/ESMTP session.
OK, so are there solutions for spam-filtering in IPv6 that allow for dropping a large fraction of spam connections to the recipient MTA before the content is transferred?