What’s the purpose of configuring additional email domain(s) in Exchange Server?
You may provide email hosting if your company has several business entities with different domain names such as .com, net, edu, etc…. You don’t have to subscribe to an email provider to host these additional email domains. Exchange Server 2010 is capable of this great functionality and the outcome - a Return on Investment (ROI).

Prerequisite: Make sure to point the MX Record to the Server’s Public IP Address.

Open up Exchange Management Console (EMC), click on Organization Configuration, then Hub Transport then Accepted Domains. In the Actions Pane click on New Accepted Domain, type the Name and Accepted Domain, click on New.

You will now see all the Accepted Domains that have been added.
For privacy reason I've crossed out the Domain Names.

Don’t forget to create an email address policy for the newly created authoritative domain. In the action pane, click New E-mail Address Policy, then choose the accepted domain for the email policy that you have just created.

Important to Know
: Use an excellent Anti-Spam protection in your Exchange Server. I would highly recommend Microsoft Forefront for Exchange Server based on my experience but Microsoft discontinued all of their Forefront products last year, you may try Symantec Endpoint Protection.

Additional Information and an Added Bonus: If your company is hosting Exchange and your websites are hosted somewhere else, you will need to contact your Web/Email Host and have them configure the MX Records. You will create a Host (A) in your Sever’s DNS Manager pointing to the correct DNS records of your Web Provider. You may also run nslookup ( the nslookup will inform you if everything is properly configured) of company1.net (just an example) to find out the correct DNS records. If you have more than one Domain, just repeat the process of adding another Host (A) for the other domains in the DNS Manager. Make sure that you’re able to access your website in your internal network. If you can’t access the websites then there’s something wrong with the DNS configurations from the DNS Manager. After the above process is completed, go to EMC>Organization Configuration>Hub Transport>Accepted Domains, you will need to add those 2 Domains, for ex. company1.net , company2.au, etc.