You can not use Open Directory in the place of Active Directory for Microsoft Exchange. You _could_ have an Active Directory implementation provide pass-thru authentication to your Open Directory KDC, but you would still need to create user objects in Active Directory that would map to the the Kerberos realm of Open Directory.
The University of Michigan has done a lot of work in Kerberos interoperability between Active Directory and MIT Kerberos. You can read more about it here.
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