From Héctor Moreno Blanco in the mailing list :

We have disabled Mail module successfully with:

  ModulesConstraints = {
      Mail = {
          uid = admin_usr;
      };
  };

However, we have found some problems with other settings. For example, when editing the categories in Preferences, we can’t modify them (create, delete, modify)… There are no logs about this, just a java error “Uncaught TypeError: Cannot set property 'controller' of null” in the browser.