If you are using Exchange Server, you can restrict who has access to the all-employees distribution list. Whatever you called it, you can prevent someone from accidentally sending a private message to everyone on the server and you can prevent people from responding to an all-employees distribution creating annoying spam. On the properties dialog for the distribution list you use for all-employees, select the Delivery Restriction tab. Change the radio button from All to List in the Accept message from section and add the accounts you want to have this capability. A better, easier to manage approach is to restrict that capability to a distribution list. Create a group you want to be able to send out all-employee broadcasts and add it to the Accept message from section rather than individuals.