The most effective place to include information about policies and best practices for email writing would be in an internal style guide for employees to follow. This ensures that all employees have access to the information in a consistent and easily referable format.
However, additional locations like the company website (for any publicly accessible guidelines) and through training sessions conducted by supervisors for new employees could also be beneficial for reinforcing these practices. Relying solely on word of mouth may lead to inconsistent understanding and application among employees, so it is not advisable as a primary method.