I would think it a worthwhile addition to include SqlGrey or PostGrey into ISPC3. SqlGrey is a Greylisting system based upon the Perl PostGrey. SqlGrey also has an add-on GUI for a web interface to control the system. Would really assist in handling the growing amount of Spam for ISPC3 users without requiring them to go through my HOWTO on the Tips/Tricks/Mods forum here.
I am pretty sure greylisting was originally planned. At least there was table in database for that. Haven't checked if it is still there. But if you are planning to add this to ISPConfig, i give my vote to sqlGrey, mainly because sql-backend. It will be easier to manage through gui.
