Why are all the RBL checks failing?
Something is wrong with the local machine's DNS resolution capability
Net::DNS::Resolver is installed and working (as I can see from further up the debug output). As far as I can tell, the RBL hosts are working at the moment.
How do I debug this problem?
What program does SA use to do the RBL checks? (and how can I run it manually?)
SA doesn't use a program, DNS resolution is an OS provided system call that any program (including SA) can use directly.
However, you can use the command line tools that do DNS resolution to test things:
dig is probably best if you have it.
nslookup or host will work too.
One thing I'd do is carefuly check your /etc/resolv.conf. Is the first listed DNS server working? try:
dig @1.1.1.1 www.spamassassin.org
where 1.1.1.1 is the first DNS server listed in resolv.conf.
