I am also interested in this question.
For starters, I did these actions:
cp /var/lib/pmg/templates/local.cf.in /etc/pmg/templates/local.cf.in
vim /etc/pmg/templates/local.cf.in
# ...
bayes_auto_learn 1
Log before:
SA score=1/5 time=3.518 bayes=undefined autolearn=no autolearn_force=no
Log after:
SA score=0/5 time=4.529 bayes=undefined autolearn=ham autolearn_force=no
Train spam by yourself. Ham is working well, but spam need to be done manually. And It's better not to tweak as original setting is very conservative not to mis-train maybe-spam, which really may be no spam.