The dovecot antispam plugin watches a defined spam folder (defaults to
"SPAM"). Instead of moving mail into special folders or forwarding
them to special mail addresses for retraining, the plugin offers two
actions for the user:

    1. moving mail out of the SPAM folder and

    2. moving mail into the SPAM folder.

The dovecot 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-clas- sify the message (as spam/not spam depending on which way it
was moved.)

WWW: http://johannes.sipsolutions.net/Projects/dovecot-antispam