RE: IP Blacklists
It's not always a Blacklist problem,  As Tom pointed out there are several
services that have lists of email addresses and the IP that any message
using that email address is supposed to originate from.  When a change is
made, like has occurred for vanagon.com, it takes a while for the
information to propagate across the net and the new IP to be associated with
the email address on those lists.  And apparently that type of Spam policing
can have some problem.  A member of another list I'm on never did get an
issue resolved sending personal email to another list member where the
receiving mail server was convinced that her mail was not coming from a
matching IP.