1. Point the mx record to the exchange 2010 server for different domain from your domain registry dns server.
2. Open Exchange Management console, Go to OC->Hub Transport->Accepted Domain->Click on New Accepted Domain to add the email address you want the exchange server accept.
3. Enter the email domain you want to add for the email address(Authoritative Domain), repeat the steps for the additional domain.
4. After accepting the domain, you can set up the email address policy so when you create new user, the user set with the email address you define. Go to-OC->Hub Transport->E-mail address Policies. Or you can add the email address for each user manually.
5. Just follow the on screen instruction, at the last screen, browse with accepted domain for the new domain you define. Repeat the steps for the additional domain define.
*You can create one one e-mail address policy for one email address, or you can use one e-mail address policy to define multiple email address under the same policy. So it’s depends if all user going to have email address for multiple domain then you will use one policy holding multiple email address otherwise you may want to create one-email address policy for each domain.