[Exim] no external eMail Users

Hi,
plese excuse my vy bad english.
I have an IP Adress and a qualivy domain name.
On my LINUX server run exim 3.16 vy well.
I do not want, that my internal users can recieve thr
eMails from a external account.
How can i forbid that ??
Please help..

Greetings
Albert

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