Cross forest groups?

I'm trying to move some exchange mailboxes cross-forest... however, I'm reading that I need to add the user doing the mailbox move "Exchange Recipient Administrators" right. However, I'm not able to add a user from another forest, even thought I have a forest trust setup. Am I doing something wrong?

"Exchange Permission requirements: Logon account for the user who is running Move-Mailbox needs to be a granted the "Exchange Recipient Administrators" role for Source and Target Forests and "Exchange Servers" role for both source and target Server. Permissions for legacy Exchange Servers remain the same as they were for Exchange 2003 Migration Wizard."

From serverfault Brett G

• This article explains quite well how to configure cross forest administration for Exchange and should get you what you want. You don't say whether your using Exchange 2007 or 2003, the article is for Exchange 2007 but the principles should be the same for 2003.

  From Sam Cogan