after fiddling around I managed to switch on a webmail service that it appears to really be turned on (before that accessing webmail.<domain>.com always lead back tot homepage of that domain).
Now the problem is: The server doesn't execute the PHP, the .php file wants to be downloaded. I am running plesk 10.4.4 on ubuntu 10.40 lts with Apache2 and FastCGI. All other instances and subdomains where I use .php work fine.
Another issue is, that the mailman app seems to work, but the webinterface is never found. Accessing lists.<domain>.com/admin/mailman leads to a not found error. Is it my DNS or something wrong with the Plesk setup?