You could check if the MX records still point at your service, or, better yet, send a test e-mail to that domain every so often and see if it reaches your service.
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... ;-)