http://www.webservertalk...e128-2006-3-1440614.html :
If you are runnig Exchange 2000 or higher in native mode, you don't need
the X.400 address anymore. The reason it was there was because in Exchange
5.5, all internal routing was handled by X.400 address rather than SMTP. In
Exchange 2000, all internal routing is handled by SMTP so you can safely do
away with it. Just unchceck the box for the X.400 address in your recipient
policies and then have the recipient update services update/rebuild all the
email stamsp on your AD users.
*use at your own risk. What I'm saying is true, but only you know your
network and only you are responsible for it.