Validate incoming email usernames against a Secret. It will allow email addresses of the form username.hash@doma.in, where the hash are the first N hex bytes of hmac<sha256>('username', secret). This allows using catch-all domains while reliably placing blame when receiving spam at one of these addresses.
recipient_filter/: A Postfix SMTP access policy delegation server that validates receiver email addresses with a secret.mailgen/: A CLI tool that generates valid email addresses forrecipient_filter.