We currently have a Server 2k3 AD domain for "firstcompany.com" and Exchange 2k7 running for that domain. We also have a second company, "secondcompany.com", which we just host email on Google Apps.I'd like to host "secondcompany.com" internally.What I'm looking for is recommendations on how to do this. I see how to allow the "secondcompany.com" domain for Exchange, but I'm looking for some other "Best Practices" for this. Should I create a second domain in Active Directory? Should I just add the other domain email addresses to the mailboxes? What's the best way to handle this in Outlook? The people that work for the second company need to primarily send from "secondcompany.com" email addresses, but they are also part of "firstcompany.com" and all corporate communications tend to come from there.

You don't need to make any changes to AD. Go into Exchange and add the new domain to the Accepted Domains and then create and Address List Policy to set that domain as the primary SMTP address for the users that need it. You will need to set something on the Address List Policy to distinguish the users. You will also need to create the MX record for the domain. Thanks Will