How to prevent a users group to send mail to internet

A

Apoli

Hello,
i have setup a FE-BE exchange 2003 mail system. I have seperated the users
into 2 groups: internal & external. All of them have access to their
mailboxes via OWA and all are able to send e-mails to internet. Is there any
way to prevent only the group Internal from sending mail to internet?
 
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