I think a relatively easy one, if the author was interested in securing this more, would be to strengthen the requirement around the "mailbox id". Specifically

I think a relatively easy one, if the author was interested in securing this more, would be to strengthen the requirement around the "mailbox id". Specifically require it in a single header, like how it's done in the example. That way, you could set up your own tests to match expectations, but it would be useless to use to abuse another third party (as the site that you're attempting to verify on certainly won't send that header).