sending email to Internet through GWIA, is there any way to allow only
messages that come from a valid user?

The problem is that in spite to have smtp relay disabled, the GWIA is
allowing the users to send messages to Internet if the sender is from
its same Internet domain.

For instance. If the gwia is configured to service the domain acme.com,
it let to send a message to Internet through smtp if the sender is
something@acme.com, although the user something doesn't exist.

Jose Luis