E-Mail is Asynchronous
The great element of E-Mail is that you read it when you want to read it. Those who check E-Mail every five minutes are using E-Mail in an undisciplined fashion. Reading and responding to every E-Mail immediately sets false expectations for those who send you E-Mail. It may be valuable to set a time within your E-Mail systems or through policy on when people should use E-Mail, thus establishing some clear disciplines on using E-Mail effectively.