You can configure restriction on adding of Exchange Mail Contaccts for your customers by following below mentioned steps :
- Login as Global Admin user in HC10.
- Navigate to menu : Configurations >> Panel Conf. >> Manage Role Permissions.
- Here specify Select User Role as Reseller/Client.
- Now under Exchange >> Mail Contacts link select check box against "Do not allow to perform 'Add' action".
This way, now your customers would not be able to add Mail Contacts via HC10 panel.