Microsoft revealed that users of Antigen have a new RBL (Realtime Block List) provider for use with Antigen products, Spamhaus is one of the most widely-known providers on the market. It’s come to our attention recently that Spamhaus has phased out the old SBL-XBL list, which has now been replaced by the ZEN list (named after a guard dog). If you’re already using Spamhaus as a RBL list in some way, you need to decide whether to use only the ZEN list, or a combination of the SBL, XBL and PBL lists. Really, if you are happy with blocking the content of all 3 lists, it makes sense to go with Spamhaus’ recommendation of using just the ZEN list. This will improve your lookup responses, as you’ll only need to make one lookup per mail, rather than a potentially 3 lookups (one per SBL, XBL and PBL list), explained.
More info: Spamhaus DNSBL Usage page