Exchange organization can not span forests - there is nothing we can do about it with Exchange 2000.
This might be your only option; use it all the same, I understand you do not have an NT 4.0 domain.
Q278888 XADM: Associate an Exchange 2000 Mailbox with a Windows NT User
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Heres my problem...
BigCompany.com aquires smallcompany.com. Smallcompany.com wants to use
bigcompany.com's Exchange Server. Smallcompany.com is set up on its own
domain in its own forest (not a child). How do I get smallcompany.com AD to recognize
bigcompany.com's Exchange server? Do I need to create a trust relationship between the two forests? Will running /forestprep & /domainprep on smallcompany.com take care of this issue??