Question : How to implement between domains of different forest?

There are 2 internal w2k3 ad domains, both are belong to different forests. Both domains are reside in the separate networks, in the same premises. Due to the Exchange server reside in one domain, the other domain users have to use this Exchange server across the forest.

So I am thinking physically linked these 2 DCs with relevant opened ports, and get the Exchange domain trusts the other domain, so as user can use exchange server. How to implement? What ports have to be opened for DCs to talk to each others? How about DNS, any zone need to create?

Answer : How to implement between domains of different forest?

Best thing to do is to create stub zones of each domain in the DNS of the other.
http://www.windowsnetworking.com/articles_tutorials/DNS_Stub_Zones.html

then once you are able to resolve the domain controllers from each forest create a forest trust between the domains
http://technet.microsoft.com/en-us/library/cc776940%28WS.10%29.aspx

the ports that need to be open between the two domains are 398 (ldap) 3268 (global catalogue) both TCP

once this is complete users can be granted permissions in each domain including linked mailboxes (i.e. cross forest mailboxes)

Cheers