I noticed that my local procmail scripts weren't running, and tracked the problem down to this:
The first lookup-domain.pl in turn tries to run a second copy of lookup-domain.pl which should be at /usr/libexec/webmin/virtual-server/lookup-domain.pl, but it seems not to be picking up the root= line in the /etc/webmin/miniserv.conf.
Here's the pertinent error in procmail.log:
Failed to run /virtual-server/lookup-domain.pl : No such file or directory at /etc/webmin/virtual-server/lookup-domain.pl line 17. procmail: Program failure (2) of "/etc/webmin/virtual-server/lookup-domain.pl"
-rw------- 1 root bin 1899 Sep 7 22:59 miniserv.conf
The miniserv.conf file includes: