Indeed, but it's added complexity and I'm not sure how many people would bother setting the DNS records I asked for. I might try it at some point, though.
This is HN, I’m sure there’s plenty of us who have spare unused domains that would do it.
I have one domain at hand that I don't use for mail at all, so people counter >= 1. I could set any MX/SPF records you want as long as I don't have to change them every two months or so... ;-)