As a general rule, you are liable for your employees' actions, so it is important to minimise the risks.
Set up clear policies - for example, providing guidance on how to use email and what content would be inappropriate. This will help to provide some protection, but only if you enforce the policies - simply stating that misuse is forbidden, but doing nothing to stop employees doing so, will provide little or no legal protection.
Ensure that compliance with these policies forms part of your employees' contracts. See Employment Contracts.
