Exchange email address when in a child domain

We are setting up a new Windows 2008 forest with child domains.
We named the top domain "" and the children domains "" and "" .  

We will be installing an Exchange 2010 server at the "" level and at the ""

Attached is a diagram of the domain layout.

User mailboxes will be located at the "" level

My question is as follows:

How can I keep the "" domain name from being in the email address? My goal is to have the email address end with "".

Example:  I have a user (mwilson) with an account and email box in the "" domain. I do not want their address to be "", I want it to be ""

I'm sure this is possible, I just want to know how to do it.
Thank you in advance!
Who is Participating?
Stelian StanNetwork AdministratorCommented:
You need to create a policy for that:

How to create an e-mail address policy:
CanHasCheezburgerAuthor Commented:
Thank you very much!
Stelian StanNetwork AdministratorCommented:
Glad I could help you!
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