Shoving letters at the end of an employee's name does not mean that person is entitled to more pay or promotions. However, if those emails are sent externally to clients or business partners then it could imply that employee has a certain level of expertise or authority in the company that she may not have. The implications of that mistaken authority will differ depending on the designation and the employee's role.
You hired her with these certifications and you recognize the certifications, don't you? If you hired her as a plumber and she answers her email with her certifications above, I'm not thinking it makes any difference to plumbing clients, do you? If company policy doesn't forbid it across the board, or in some other way, monitor certifications, I'm inclined to let her do it.