When/How does a new accepted SMTP domain get added to
Exchange 2007?

I am trying to troubleshoot why step RUN.15 - Create a Reseller Organization
Admin Using Provtest is failing for me (I have a specific post for that thread).
 My AD domain is different that the email namespace I will be using for the
reseller.  For example, my AD domain is hosting.local, and the reseller email
namespace will be reseller.com.

Step RUN.15 has me creating a new user account called admin@reseller.com,
heowever, Exchange does not yet have reseller.com as an accepted SMTP domain. 
How can this account be created without the reseller.com SMTP domain?  

I have looked through the documentation and the only place I see it mentioned is
in the Configure DNS for the Reseller Organization.  The description of the
section says "...and an accepted Simple Mail Transfer Protocol (SMTP)
domain must be set in Microsoft Exchange Server 2007."  However, the
section does not walk you through the process of creating the SMTP domain.  

Am I missing something?  At what point is the SMTP domain created?  If it is not
created, would this cause RUN.15 to fail?