Software Suggestions for set up of internal webmail server

One of my clients would like me to set up an internal email server for a group of about 125 employees. They would log in via webmail and access would be only through the client’s internal network.

This would be put on a Windows 2008 server.

Have any of you done anything like this and what email software did you use? I also manage their web server so another option could be putting something on the web server (Windows 2003).

Thank you in advance for your suggestions!