Response     You may modify it as you wish, but keep it short. It will probably be sent to very many forged and/or non-existent addresses. You want to add to the email traffic as little as possible. Do not use + or -, and do not change the email address shown.

Sorry, your message has been rejected because your address is NOT on my list of allowed senders. You can put your address on the list by sending an empty message to mark.grey+77799xx@fictitious.domain.go Then, your other messages will be accepted normally.
both, the code and response, on the server.