RBL Checks Broken

[ISP List]

I'm struggling with a problem that is driving me nuts.  Here are the details.

MailScanner 4.12-2
SpamAssassin 2.44
FreeBSD 4.7
exim 4.10

The crux of the problem is that I can't get SA to run any RBL checks.  I've
made sure that "skip_rbl_checks" is *not* set to 1 anywhere and that the
RBL checks have a positive score assigned to them in 50_scores.cf (note
that I am not trying to use any paid RBLs, just the free ones that are
enabled by default).  I've eliminated all user_prefs files that might be
lurking about, and even explicitly set "skip_rbl_checks" to 1 in
/opt/MailScanner/etc/spam.assassin.prefs.conf.  I'm using an out-of-the-box
SpamAssassin and MailScanner (that is, no modifications to the code have
been made).  I have one machine that *will* run the RBL checks, and one
that won't, and as far as I can see they are identically configured.  Any
pointers on where to start looking?

--Mike