Net::DNS 1.01 has an undocumented non-backwards-compatible (but, according to
the author, intentional) change: the "recursion desired" bit isn't set by
default in new packets. However, SpamAssassin relies on this when constructing
RBL queries.

diff -x config.log -x config.status -ru tmp/Mail-SpamAssassin-3.4.1/lib/Mail/SpamAssassin/DnsResolver.pm work/Mail-SpamAssassin-3.4.1/lib/Mail/SpamAssassin/DnsResolver.pm
--- tmp/Mail-SpamAssassin-3.4.1/lib/Mail/SpamAssassin/DnsResolver.pm	2015-04-28 20:56:49.000000000 +0100
+++ work/Mail-SpamAssassin-3.4.1/lib/Mail/SpamAssassin/DnsResolver.pm	2015-07-15 20:53:55.594508162 +0100
@@ -575,6 +575,7 @@
                 { $1 eq '\\' ? "\\$1" : sprintf("\\%03d",ord($1)) }xgse;
 
     $packet = Net::DNS::Packet->new($domain, $type, $class);
+    $packet->header->rd(1);
 
     # a bit noisy, so commented by default...
     #dbg("dns: new DNS packet time=%.3f domain=%s type=%s id=%s",