Thank you for your answer. I'm looking about the threads in the forum talking about spamd. This could be one problem I have here.
I heard from other sides that there might be also a performance problem outside ISPConfig. The combination I installed (vmware server on a host with drdb drive and Fedora Core 6) might have also performance problems. So I try now to install a FC5 system and do some speedtests first. If they are better, I install ISPConfig on this system and also try to solve the problem with spamd.
I think this system must be able to handle a few hundred email addresses without problems. So I was surprised about the delay and load I have in the VM and also on the host. it was permanent over loaded.
Thanks again for your hints.