The Microsoft Forefront Server Security Support Blog points out that Spamhaus has recently phased out some of their RBLs.
It’s come to our attention recently that Spamhaus has phased out the old SBL-XBL list, which has now been replaced by the ZEN list (named after a guard dog).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’ve got Spamhaus configured in your Exchange connection filtering you will want to be sure you are not using one of the discontinued services. Short story is that sbl.spamhaus.org and xbl.spamhaus.org are no longer in service, and users should switch to zen.spamhaus.org. See the Spamhaus website for more details.