Is there any way to prevent internal users to send Spam to Internet using Astaro?

Is there any way to prevent internal users to send Spam to Internet using Astaro?
Astaro is accepting relay only from mail server, but some infected machines are sending ton of spam to Internet through mail server and Astaro.

The internal machines are using dynamic IP´s.

Thanks in advance.

Best Regards.

Sandro Issufo

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