Have you tried with standalone exchange mailbox or Cross-forest?
DeVa, M.S., {MSFT}
How i found the function GetRights has the following signature
GetRights(ENTRYID UserEntryID, ENTRYID EntryID, out int Rights);
the first param (PR_USER_ENTRYID) is read from private IMsgStore.
for the second param i used PR_USER_ENTRYID of other user or PR_ENTRYID of other user's IMsgStore.
it always returns the error SyncInvalideParam.