[exim] Allowing SMTP from localhost only

Hi!

I have a Debian Sarge box with exim4 set up for "local delivery only".

Because a web application I am running there needs to be able to send
authentification and reminder mails I need to modify this config slightly
to allow delivery of mail to remote hosts. For security purposes, however,
I want this to be allowed from localhost only!

I have skimmed through the configuration file and the documentation but I
could not figure out how to achieve this type of configuration.

Any help is greatly appreciated!
Cheers, Peter