Inside of exchange system manager, open up the recepients then recepients policy.
here you will see the default policy. this automatically gives any exchnage user your default domain SMTP address. if you want to add additional domains, you can either modify the default rule, or add an additional rule. Under the email Address policy tab, click new and add the ip address you want to respond for (i.e. @fubar.com). the rules are processed in the order they appear. as you add rules they take a higher precedence. When the Recepient update Service is run, it courses throught the list of recepient policies, and applies them based upon their rules. once a user object is matched with a policy., the user object is not processed against any other policies, hence the top down processing.
if you want to assign a smtp address to only a select group of users (perhaps hosting) you will have to have some type of consistent value to check. I have not been able to get rules based on OU= working. I use logon name instead. in order to accomplish this you will want to add a UPN for each different domain you will host.
To add a UPN, open up active directory Domains and trusts. right click on the domain, and choose properties. here you will be able to add a upn (i.e. fubar.com). Once you have added a UPN, you can now create users that have the UPN name instead of your default domain name (i.e. bgates@fubar.com instead of bgates@microsoft.com)
back to the recepient policy......
on the filter rules, choose modify/add and from the "find" drop down, choose custom search. Click on the "Field: drop down and choose user/logon name. Then chooses "Ends with" as the condition, and the value "fubar.com". this would set up an email assigning policy that only applies to fubar.com users. If you did not want the default domain SMTP address to get assigned to these users, you would need to set the new address you assigned as the primary, then uncheck or remove the default domain SMTP address.
Hopefully this wasn't too confusing.