Mailbox Folder Permissions Issue

Hi,

I am having issues with an individual user.  This user is located in a separate forest and is the linked master on a mailbox in my exchange forest.  The user has permissions on folders that appear as "NT User:domain\user" and everything works (FYI this is an Exchange 2010 environment, upgraded from 2003).  Problem is that when I try to add new permissions, or if I delete the NT User entry and re-add the permission, the user no longer has access.  The domain that shows up in "NT User:domain\user" is not the domain in the resource forest. 

Thanks

Corey