We’re now using the dbl.spamhaus.org blocklist
The Domain Block List is an extremely reliable list of domain names that are used only in spam. Blocking most mail that advertises these domain names improves our spam filtering: we’re now blocking about 1% more spam as a result.
That may not sound like much, but it represents about 150 more blocked spam messages per year for each customer. (We block an average of over 15,000 spam messages per year per customer.)