this sounds really clever.

One suggestion - external contacts won't know that this address will bounce for anyone they add in CC. Maybe have an option where if I include the original whitelisted domain/person in the chain then it will let the mail through even if it's from a non-whitelisted person.

Just thinking of scenarios like, you give out your email address to <your-neighbour@gmail.com> who then emails you about a 5-a-side game with a few friends but any messages from any of the other friends bounce.

Or you apply for a job at ACME Inc and then you get a message back from `alice@acme.com` saying, please coordinate with `bob@our-recruiter-consultancy.com` who'll arrange the interview but Bob can't contact you