That is core of the problem. I didn't make any of manual changes. I upgraded MySQL to 4.1, and installed webmin (but didn't changed anything using webmin), and added deleted few times /etc/postfix/virtusertable (below line for manual entries) I had a lot of problems with ISPConfig please take a look at: http://www.howtoforge.com/forums/showthread.php?t=2557&page=2
