An Exchange domain issue

Hello,

I have an Exchange 5.5 mail server. It is configured to create mail boxes of type: account@domain1.com

I would like to configure the same mail server to create mailboxes for another domain i.e. account@domain2.com

I have configured my DNS for the domain2.com domain but which option will enable my Exchange 5.5 in order to provide mailboxes for both domain1.com and domain2.com?

Thank you in advance.