[Hans]

That looks ok..But i asume that you've enabled Maildir then within ISPConfig as it is not enabled within your etc/postfix/main.cf file with the line:
home_mailbox = Maildir/

When i open a telnet session to your mailserver with telnet server1.grikservers.com 25 , i can not establish a connection.

So, i like to know what is the output of: netstat -tap