I doubt anybody would consider using Marketo for internal email if they can't get test emails from Marketo anyway!
And anyone who configures such a system should be forcibly retired. It's one thing to fail to whitelist Marketo-originated emails (whitelisting can be done totally safely by whitelisting DKIM-aligned emails from your domain only, though it is rarely done). To outright blacklist Marketo-originated emails as if they are forgeries is just stupid.
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