ok i realized it. The "from:" Mailheader ist set by squirrelmail in his own config.php by "$domain". If you leave it empty the /etc/mailname is used, cause of "myorigin=/etc/mailname" in postfix.
So the only thing that works for me is to copy squirrelmail to each virtual domain
and set the "$domain" in config.php. ....
Now the problem is the username. Here i can only delete the prefix in ispconfig, but then i had to use the whole username, to avoid conflictions with same names, if hosting in different virtual domains ..... andwhat is with "hans-mueller"
----- so i forget this and use the prefix.
For first ;-) its ok, when a new user send his first mail with squirrelmail and only the username is set to web_x and the domain is right.
But what is if using Thunderbird - oh my god - he have to fill in his right name and the right domain and user with prefix like web_x for smtp/imap server........ ( excellent, that you have to fill in the username/adress in thunderbird, instead of be able to overjump this)