Microsoft Exchange 2003 Opening another user's calendar in Different domain

How do you open another user's calendar in a different domain?  i see the user in our gal but when I add their account to the mailbox and give them permissions, they receive unable to open folders?

The firewall has been open
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There would need to be a trust setup between the 2 domains in "Active Directory Domains and Trusts" on the domain controller.
cndybndyAuthor Commented:
What if the exchange servers are in the same Domain but different Forest?  For instance both units login into 123 however our Exchange Servers are named TwoC and ThreeC.  I can't see the users nor can they see me.  However, if I have an account on both Two and ThreeC could I setup Outlook thru the ThreeCs proxy server and my mailbox on that server?
cndybndyAuthor Commented:
We had to open the firewall for two way trust
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