[Exim] Problem with relaying

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It seems I need a new rule in my exim4.conf file to prevent something from
happening...

I am using exim 4.12 with sa-exim 2.2 on a Linux 8.0 machine....

The situation is this:

A machine from the internet is able to connect to my mailserver, give a from
address that appears to be from my local domain and can send to anywhere
else on the internet, thus making me an open relay of sorts.

It gives the correct message when they give a non-local address for the from
address and a non local address of "relaying not permitted" however, so I
know something of some sort is working.

How do I enter a rule in my conf file to fix the above listed problem?

Thanks for you help!

-Tom

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