Do I need Exchange Server for managing 6 or 7 users?

Hi

We are a small charity organization, have small network ??? Client / Server based network.
We have 6 workstations & 1 server (which acts as a Domain controller). Currently we are using our Isps Pop3 mail account configured in Outlook. We are thinking to get an Exchange server for managing emails. My question is it worthwhile spending money for an ne exchange server for only 6 or 7 users in the Office.