Adding additional mailbox from a different forest in Outlook

[astaylor]

Background (Quick):
Merging multiple companies together to a common domain and Exchange server. Transition is not overnight and just starting with 1 company for now. Only a few users on the new Exchange server at this time.

Environment:
OldDomain (1st company): Server 2003, Exchange 2003, Outlook 2003/XP
NewDomain: Server 2003, Exchange 2007, Outlook 2003
2-way trust established between both forests.

Problem:
Without going into too much detail. In Outlook a user that is on the OldDomain needs to be able to add an additional mailbox (not mail profile) for a mail account that is on the NewDomain.

I am able to configure Outlook to soley work with the NewDomain mail account on the OldDomain so i know permissions are correct.

When i go into the additional mailboxes section to add the new mail account it does not show in the GAL (obviously since it's only showing the OldDomain accounts). Is there a way to accomplish adding a mailbox other than from the GAL in a way that i can use domain/username? Maybe through the registry? Or maybe, is there a way to have the GAL from the NewDomain show in the OldDomain as well?

Any help would be great.

Thanks

-drew

MCSA 2003: Messaging