configuring sendmail

[Sergio Matos <smatos@hdfaro.min-saude.pt>]

I have installed sendmail to function as a internal mail server for
my intranet. I think sendmail rejects mail for the internet by
default. I do want to reject mail sent to the internet but I want
to send a error message to the sender.
If someone knows if this can be done, and how, I'll be grateful.
 
Thanks,
-- 
Sergio G. Aleixo de Matos

Universidade do Algarve
UCE - Campus de Gambelas
8000 Faro
Portugal