I see people using defunct RBL/ORBS/ORBZ zones, which contain poison:

;; ANSWER SECTION:
194.24.183.199.orbz.gst-group.co.uk.  25w5d IN TXT  "This zone is defunct stop using it."

  As a result, those using them are bound to have all their incoming email
(or a sizable portion, there is) becoming rejected irregardless if there
is any real _RBL_ on address, or not.

Carefull out there folks,  keep an eye at your configurations, especially
if you are trying to use "free" RBL-like databases.  They appear to have
limited lifespans.

/Matti Aarnio -- co-postmaster of vger.kernel.org