Spam scanner needs training. We'll occasionally have false positives and
false negatives. Now this is the point where the Dovecot antispam plugin
comes into play. Instead of moving mail into special folders or forwarding
them to special mail addresses for retraining, the plugin offers two actions
for the user:
* moving mail out of the SPAM folder
* moving mail into the SPAM folder
This plugin watches these actions (and additionally prohibits APPENDs to
the SPAM folder, more for technical reasons than others) and tells the spam
classifier that it made an error and needs to re-classify the message
(as spam/not spam depending on which way it was moved).
