Having Exchange use different DNS servers to resolve names for *outbound*
mail won't have anything to do with spam, NDRs, etc....
No prob. If you're getting a ton of spam, get something that can keep it
from getting to your server in the first place. SpamAssassin, GFI,
www.postini.com - there are many options.
In addition, see www.vamsoft.com for the Open Relay Filter, which is a swell