Error 404 - Not found - The document you requested is not found

I have been creating the DNS records for a second domain that will be pointing to the same IP address where the Web Site is being hosted.
The current domain - let's call it mydomain.com is working perfectly well, email is going to a In house Exchange server and the website is hosted with ISP.  No problems.  Then we purchased a new domain name -let's call it domain.com  and the DNS entries have been made.  
We are using Small Business Server 2003 R2
I created two host (A) entries:
@  - pointing to the ISP IP address where the website is being hosted
A  - host name is Portal   pointing to External IP of the business Location where the SBS Server is
I also created
www  as CNAME for @
MX  record pointing to Portal (being this the host name of the A entry made poiting to the SBS Server)
The problem is the following:
I use mydomain.com or www. mydomain.com and I get the webpage with no problem
I use domain.com or www.domain.com and I get this error message.
I would like to find out if there is an error in my DNS Entry or is it a problem with the website hosting or code in the webpages that need to be addressed?
All the other entries are working with "domain.com" domain name.
Please let me know.