To build a simple exchange server, then you may want to install exchange 5.5. This version had no major integration with some major Microsoft components.
Here a good decent instructions:
I would not recommend Exchange 5.5. It is out of support with Microsoft and you will not be able to get help with it should you need it. Same with Exchange 2000. Exchange 2003 would be the simplest solution, but if you have the hardware capability Exchange 2007 would be more powerful. Depends on how many people you will be hosting mailboxes for and the trouble you want to go through. But I would never recommend a product with no support for it. What operating system will you be installing? Please provide more details. Thanks.