Microsoft Knowledge Base Article 304935 contains the following summary:
This article shows you how to programmatically modify mailbox rights on a Microsoft Exchange Server 2000 or 2003 mailbox at the same time
that you mailbox-enable the user object in the Microsoft Active Directory directory service.
This article includes sample code that shows you how to set the mailbox rights for an Exchange 2000 or 2003 mailbox before the actual mailbox has been created for the user in the Exchange 2000 or 2003 information store but after the user object has been mailbox-enabled in Active Directory.
Note This code has no effect if the mailbox already exists in the Exchange 2000 or 2003 information store. In other words, it does not affect the actual mailbox rights on the user's mailbox if the user's mailbox has already been accessed.
For more information about how to mailbox rights on an Exchange 2000 mailbox both before and after it has been created in the information store, click the following article number to view the article in the Microsoft Knowledge Base:
310866 How to set Exchange 2000 mailbox rights on a mailbox that exists in the information store