Exchange Server in Mutiple Forests

Hi,

Kindly help me in the following scenario:

* I have two forest on my network with two domains called Admin and DB1 (both are DC).
* Now I have installed exchange server on separate server machine called Exchserv1 which is client member of Admin domain.
* Now the AD of Admin is accesible from Exchserv1 and all exchange tasks are being performed.
* But I want to perform exchange tasks on DB1 domain aswell but when i try to create mailbox of the user of DB1 domain there is no mailbox server shown in the list.
* I have run forestprep on both domains and there is also a external trust establish between two forests.

Kindly help in this regard.