The dovecot antispam plugin is aimed to help in spam systems training.
Consider the following scenario. Your mailbox is flooded with tons of
mail.  They are delivered to the configured folders based on the
arbitrary spam system judgement. This judgement is not always correct
or not always what you would like it to be, however. This is where
this plugin comes into play.

All you have to do is move the mail into the correct folder. All the
rest in order to let the spam system know it made an error will be
done automatically by this plugin.

It watches for when you move the mail to some folder and acts
accordingly.  The advantage of this approach is that the mail ends up
in the right target folder directly and needs not be touched twice.

WWW: http://hg.dovecot.org/dovecot-antispam-plugin/