Why is it not valid for someone to send email from another domain? I’m asking seriously. That’s not an invalid thing to do. The RFC explicitly advises that flexibility in the envelope address is a necessary feature of internet email.
It may be a convention that transactional email services require proof of domain ownership, but that is a convention, not a standard.
It may be a convention that transactional email services require proof of domain ownership, but that is a convention, not a standard.