unable to configure mailbox for tree Domain

Hi,

I have a Exchange Infraestructure working. The IT manager ask me to create a new domain and create mailboxes using actual Exchange Server.

So i create a tree domain in the same forest  and configure a trust relationship. I can see the user in exchange but im unable to create the mailbox.

Domain A: local.a.com
  Exchange Infraestructure Server1 CAS/HT

                                        Server2 DB

Domain B: i.local.com

Any help.