That's their issue. Don't be a customer at such an email provider :shrug:

I had a legal dispute once where a shop didn't provide warranty on a product. They took the product in but then it was indefinitely in repair. Long story short, it ended up being helpful that I have email server logs that show every transaction, so that I could say "on date+time the email was delivered to IP address X, which was registered as the mail exchanger for your domain, and your server took the email with status code XYZ Queued <ID> For Delivery." I got the money back after that

Afaik that's not what Microsoft/Hotmail does though: since it rejects, you get a delivery failure notification