Exhcange 2013 / Exchange 2010 - Cross Forest User Mailbox Access

Hello All -

Trying to setup cross forest mailbox access.

We have 2 environments:

1) Exchange 2010 RU13, Resource Forest/ Mailbox Forest

2) Exchange 2013 CU4, Standard Exchange Install (all mailboxes in same forest)

I am unable to get to a user in forest 1 (exchange 2010) to be able to access a Mailbox in Forest 2 (Exchange 2013). I used this command:

Add-MailboxPermission -Identity (Exchange 2013 Mailbox) -User domain\username (Exchange 2010) -AccessRights FullAccess -InhertianceType All  

When I then add the mailbox into my outlook, I am prompted for credentials, which I supply (exchange 2010 user), and then the mailbox is successfully added, however I am unable to expand the mailbox.

I get the following Error:

Cannot Expand the Folder. The set of Folders cannot be opened. Microsoft Exchange is not available. Either there are network problems or the server is down for maintenance.

Thanks,

Robert

Robert