Today, i have changed my howto at URL http://www.howtoforge.com/roundcube_webmail_ispconfig
With version 1.1 of my howto, i use the original main.inc.php file provided by Roundcube itself.
For using RoundCube on our ISPConfig server, we only need to make some modifications to this configuration file as described at step 6 within my howto.
To save some time, you can also download the main.inc.php file here.
The result is that from now on, we only can login with the username and not with firstname.lastname@example.org
The reason: to avoid the "saving identities problem".
With the new configuration file, spell checking is enabled!
Thanks for your feed back!
Doing so, we can make things even more better!