Edit: Nevermind, I see that you've already done this.
It might be worth it to pony up for an M365 license or two, send yourself an email, and then open a support ticket inquiring why the email was blocked. I would even avoid mentioning that you are the sender. Just pretend you're a regular customer who receives email from your domain and you're wondering why it was blocked and if there's anything that can be done to stop it from happening.
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Edit: Nevermind, I see that you've already done this.
It might be worth it to pony up for an M365 license or two, send yourself an email, and then open a support ticket inquiring why the email was blocked. I would even avoid mentioning that you are the sender. Just pretend you're a regular customer who receives email from your domain and you're wondering why it was blocked and if there's anything that can be done to stop it from happening.