Email system for an educational institution

Hello, i need to setup an email system for an educational institution, so that the end users can use this email service. The website of this institution is hosted @ hostmonster.com . Now is there any service that can be installed at hostmonster? Something like microsoft excahnge server? But i guess exchange server is very expensive, so i am looking for an open source or low cost solution. I dont have any kind of hosting hardware. So please help me out. Thanks.