Webmail link does not work when there are no persmissions for sites module
In the webmail.php in ispconfig/interface/web/mail is the following piece of code:
//* Check permissions for module $app->auth->check_module_permissions('sites');
that's why it's not possible for email only users to access webmail from ispconfig interface. I think it's not nessecary to have "sites" persmissions to access webmail.