thank you robilaur.
Though I'm actually not looking for instructions on how to discard any spam.
What I am looking for is some documentation or explanation on what is configured by ispconfig so I can change that if it doesn't work.
I've done some testing yesterday and it seems anti-spam tools are working as long as they are policies on e-mail domains. I was trying to force policies onto e-mail boxes. The latter just doesn't work. Changing the spam policy to not filtering at all on a box has no effect either when a domain is filtered. Quite annoying since that way it's more or less impossible (at least through the ispc system) to non-filter a postmaster or root account on a domain that's filtered.