A Web Based Solution to Replace Exchange?

benthemeek asks: "A friend of mine works for a company that has more than 6k users connecting to a Outlook exchange server instance through VPN from various homes all across the country. The executives at his company would like to move to Active Directory and a web based solution for these users. When Outlook Web Access was priced out, it was judged very expensive and they opened the floor to other options. They want a LDAP enabled, web based email and calendar that could hopefully plug in or replace Exchange, and if the solution can be load balanced between more than one server to ensure reliability and uptime, that would be even better. Slashdot readers come from many walks of life and I am sure some of you have gone through a similar experience and could give some insight to this problem. The fan boy in me would like to see a complete Open Source to meet this need, but that may not be possible. Have any of you done similar migrations, and to what solution did you go to?"