2 forrests 1 exchange with single sign on

We are merging with another company.  The big question is, can we keep our AD environments separated (but with trusts), yet use 1 of the existing exchange environmnets.  

Company A = ham.com

Company B = cheese.com

Cheese.com is going away.  All employees of the combined company will get mail from ham.com.  Can company B use company A's exchange server without doing a domian migration (just trust the forrests).  And can we do it with single sign on or will company B have to login to outlook with different username\pass (from Company A's domain).
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Matthew MillersCommented:
Yes, just add the remote forest user objects as the associated external account of a disabled mailbox enabled user in the forest with exchange.

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Matthew MillersCommented:
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Windows Server 2003

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