I'm just guessing, but I'd say you might need to prefork a few hundred kids to deal with the botnet swarms that invariably come looking for your valid RCPTs. That may not be the problem, but it's definitely a possibility — maybe a deadlock? What platform are you in? I've had some success with the milter-spamd combo and doing it from procmail on a mass scale... Perhaps spampd isn't ready for primetime? I never tried it, no idea...