I have "bayes_auto_learn 1" set in local.cf. I thought that means that bayes database should be updated automatically.
But I can see changes of the bayes_* files modification dates only when I use sa-learn utility and never automatically during the spam filtering process.
Did I understand something wrong?
Yes and no.
bayes_auto_learn will enable automatic learning. However, autolearning is a somewhat rare event, and if you're dealing with a low volume of mail (under 1k a day) you may not see it learn anything for several days.
SA is in general quite paranoid about autolearning. It requires a lot of conditions to be met to be certain it's not mis-learning.
The biggest regulator is that autolearn must not strongly contradict the existing learning (ie: don't learn a message as spam if it would have hit BAYES_00 previously, or one as spam that would have gotten BAYES_99).
Spam learning also has several extra restrictions
at least 3 points of header test score, regardless of what your threshold is.
at least 3 points of body test score, regardless of what your threshold is.
Your configuration can also impact the learning rate. If you've got RBLs disabled, autolearning as spam is nearly nonexistent, since for spam learning SA is going to require 3 points of header tests. Without RBLs there's not often that many header points (it can happen, but it's very rare).
