Well thank you Parsec for the suggestion that installing courier-mta should not be too disruptive. That gave me the confidence to try it out and sure enough it was easier and less time consuming for me than trying to work out this problem with postfix + maildrop.
If anyone happens to be interested I could write up a short guide on how I did the conversion and a few tweaks to make sure courier-mta works with a ISPConfig on ubuntu (13.04) system. I now have Dspam working as I would normally set it up and now my spam has dropped to nearly zero :-)
Last edited by markc; 12th April 2013 at 10:25.