SMTP 4xx and secondary MX record as a way to redirect mail
Solution 1:
Well, in case anyone is interested:
Yes, it's possible to implement this scheme without writing any milters: just use check_recipient_access with any custom response codes and messages in access(5) table.
And no, it doesn't work. At least GMail retries the highest priority MX over and over when it sees 4xx error instead of trying the next one. For now I ended up with rewriting envelope sender addresses with pcre type canonical table to [email protected].