I just had a flash.
In another thread somewhere today, I asked about training the spam system by feeding it lots of spam that ended up in mailboxes that were never advertised in my domain, but were for valid local users (I shared the machine with a group that used it for Samba storage).
So what about this idea? Pick non-used names for mailboxes in your domain that are likely targets for dictionary attacks, and alias them onto the spam training account.
Instant free spam training.
If you commercialize the idea, I want a free license. :-)