In Exchange 2007 you would create a
Mail Contact in the
A mail contact is described as
A mail-enabled Active Directory contact that contains information about people or organizations that exist outside an Exchange organization. Each mail contact has an external e-mail address. All messages sent to the mail contact are routed to this external e-mail address.
So I think this should be fitting your needs since you seem to look for a 1:1 relationship between email@example.com and firstname.lastname@example.org and don't want e-mails to be distributed to multiple users.
(maybe someone with experience in Exchange2010 should verify this)