6.5.1 and Window 2003

We are trying to bring up a 6.5.1 server within our domain and are having several issues. Foremost is that once we have the server up we have several sites that are unable to sign into accounts using web mail. We have no issue locally but we have several sites that are having this issue. Our older servers running 5.12 are not having any issues with the webmail access. Any ideas??? IT seems there may be a 2003 setting we are overlooking.