I am not sure that anyone will be able to help you, I have talked to everyone in my office and nobody actually knew the answer as this is something nobody should try. Everyone agreed this is 100% not the way operational e-mails are supposed to work. Operational e-mails should be used to report outages or other items that affect the operation, not to get around the fact that your clients have chosen to not receive sales e-mails from you. I'd imagine this could get you in some hot water in various jurisdictions which is why I suspect that Marketo is not allowing operational e-mails through sales insights.
It sounds like other people have had the same question. I found this discussion that should be able to help you with understanding the process:
Hopefully this helps!