It's best to make your email rules available and well known:
" Email is a quick, cheap and easy means of communication. This makes email a great business tool, but at the same time a potential threat for employers. Email threats such as confidentiality breaches, legal liability, lost productivity and damage to reputation cost companies millions of dollars each year.
How can a company protect itself from these threats? The first step in securing your company is to create an email usage policy. After you have created your email policy you must make sure it is actually implemented. This can be done by giving regular trainings and by monitoring employees' email using email security software. "
You may find it useful when trying to develop your own E-mail policy.