*** Updated by Wrobinson on 2/29/08 ***
You should allow the Active Directory installation wizard to install DNS for you. Before hand, be sure to assign a static IP address to the server itself because it is recommended practice that all domain controllers and DNS servers use static IP addresses.
Though the domain UPN suffix is used to generate the e-mail address in the default recipient policy for Exchange, you can add additional SMTP name spaces that can be configured to apply to different users based on attributes that can be queried in AD such as department, city, etc. To do this, you need to configure additional recipient policies and configure the query in advanced properties. You can even see how the policy will be applied before applying it.
*** End Update ***
1) in your NIC settings check to see what your DNS server settings are. if that server cannot resolve it then it must know where to go on the internet to resolve it.
2) you can have USERA@abc.com and USERA@xyz.com. just go into active directory users and computers, right click the user. go to properties. then go to exchange email addresses. this should list what they already have. click on new or add and then add the SMTP address of the other one. now the emails for both addresses will show up in the inbox of that one user. whichever one you set as primary is the one that will show up as their REPLY TO address when they send emails out.
You can also setup recipient templates that will give certain users one e-mail address “stem” and others another. This may actually be what you are looking for with your second question.