I wrote something in node.js that does exactly this. But I have only used it for personal use. I'm honestly surprised that nobody has done this already.

Right now it just silently drops expired addresses. But it is so satisfying to think about bouncing (but stuff like bouncing behavior is something you have to consider when running a mail service).

I was thinking of turning it into a service, but I'd have to read up on how to scale it. Running an SMTP server takes a lot of care. I've found that just using nearlyfreespeech.net's mail forwarding is most reliable to receive emails. So I do that for now, since it is on a small scale.

I just got so frustrated with how this problem has such an obvious technical solution. At least for us users, anyway. It's not a solution for the marketers.

I strongly suspect that Google is very reluctant to do anything to make the email landscape unstable. I think that if Google started offering this, it would shake up so much of their business.