[Exim] help with relay

Hello,

I'm use exim 4.01.
I has adjusted exim so as users from local network and sevaral hosts can
send email (hostlist relay_from_hosts = 127.0.0.1 : 10.0.5.0/24 : ....).
How to configure exim that users from internet can send email. But I don't
know how do that only registered peoples can send emails trought me.

P.S. thanks for any help

Bye.