[Exim] routing algorithm (smarthost and Internet)

    I installed exim 3.35 in a Debian 3.0 GNU/Linux server.
    I want to use the following routing algorithm:
    when any email comes to Exim it forward to an machine (smart host)
    but if the emails comes from this machine (smart host) the Exim routes to the Internet.
    How I can configure this with Exim?
    TIA,        Paulo Henrique