Cross-forest access to public folders Exchange 2013-2007

Dear.

We have an Exchange 2007 org in one forest and an Exchange 2013 org in another forest.
User accounts remain in the 2007 AD, mailbox moved to Exchange 2013 in the other forest, so a linked mailbox.
What do I need to do in the Exchange 2007 public folders to give the migrated mailboxes (not migrated users) access to these public folders?

Thanks for the support.
Regards.

Peter Van Keymeulen, IT Infrastructure Solution Architect, www.edeconsulting.be