Is money earned from private work that was completed using the company's computer and by sending emails from it lawful or unlawful, knowing that the partnership with the employer is 10% and not a monthly salary? And what should be done?

The work tools are a trust in the hand of the worker, so it is not permissible for him to benefit from the work device except for the interest of the work and what he has been explicitly or customarily permitted to do. If what you did was not authorized, then you must repent by regretting it and resolving not to return to it. As for what you earned from sending these emails, it is lawful for you if it was in exchange for permissible work.