Ok so I was making confusion... I was thinking that mail.domain.com was a subdomain of domain.com.
Anyway now I've created another domain: mail.domain.com and copied in the folder /var/www/mail.seatel4004.com/web the roundcube files.
Now I can see the roundcube page, but I've a database error.
I tried to use the same database that was using before.
I tried also to create a database with ISPConfig (c0roundcube). I gave to it the privileges:
GRANT ALL PRIVILEGES ON c0roundcube.* TO roundcube@localhost
IDENTIFIED BY 'password';
and of course in /var/www/mail.domain.com/web/config/db.inc.php I've put the right user and password.
Well, I still have as error DATABASE ERROR: CONNECTION FAILED!
Am I wrong in something?