Adding a second e-mail address ?

You can only have one Primary email address per mail box, but you can have lots of secondary proxy addresses. What this means is that you will have to add the second address to everyone who needs it. Exchange will not automatically do this for you.

That being said. There are two steps to this process.
Goto Site->Configuration->Connections->InternetMailconnector, Properties,
Go to the routing tab, select Reroute smtp, add your new domain name, and your current domain name if it isn't there.
Stop and start the IMC.
goto the recipients container and open a mailbox, on the email addresses tab click new, select internet mail, add the new email address.

You should be able to receive mail for the second domain name (assuming the mx record on the internet is point to your server).

Hope this helps, let me know if you have any more questions. I do this because we have had several acquistions, and people want to keep receiving emails sent to their old addresses, just in case.