Exchange 2010 accessing mailbox on different domain
I have a flat network with two domains that have a trust between them. A curriculum domain and an admin domain.
I had Exchange 2003 on my curriculum domain and used to use "Mailbox Rights" to allow the users who logged into the admin domain to access and use their mailboxes. I have recently migrated to Exchange 2010 and the people who log onto the admin domain are unable to send email. They can receive it and view emails but when they try to send they get a message saying that they don't have permission to send on this persons behalf. If they use OWA and log in with their curriculum account it works fine. "Mailbox Rights" seems to have disappeared on the 2010, how can I allocate these rights?
I have looked at "Manage send as permission" but it only displays users on the curriculum domain.