I have nearly the same setup as defined in upper blogs.
but I have windows 2003 server and exchange server 2003
1: I took the live Ip
2: create a primary zone with my ourcompany.com
3: create CNAME, and MX record entries likke
MX: webmail.ourcompany.com 
I am sure that some configuration is missing...
looking for your urgent help in this regard.