exim aliases filtering spam

Our aliases are spammed like any other account, but filtering on them would require making a real account. I wanted to be able to filter aliases without creating accounts for everything. My first solution was to create an account and filter on that one, then using $original_local_part I could forward to a filtered alias. This works but if someone discovers the filtered alias, they can bypass the filtering.

The exim docs suggested that I could have filtering in the aliases but it didn't seem to work and I kept getting this in the logs:

error in redirect data:…