You'll need to set up SMTP namespace sharing. Here's an article that explains how to do it in Exchange Server 2003:

http://www.msexchange.org/tutorials/exchange-2003-smtp-namespace-sharing.html

Exchange 2003 internal to external
I am reading a nice book on DSP 


  The Scientist and Engineer's Guide to Digital Signal Processing


It mentions 


  In electronics, the mean is commonly called the DC (direct current) value.


How can a signal have no DC component if the mean is the DC component? Surely every signal's values can be averaged?
