Email is the primary way of communication. Small organization which hosts their website / any other application from a third party hosting service providers may require this email stack.
Email is most important in any organization. Lot of business are happening via Email. This article discusses about the end to end complete Email solution for an organization. What an organization requires? Email spam filtering, Email Server, Email Archiving and Email client. If all are open source then complete solution could be achieved free of cost.