$msg = qq{From: Mailer-Daemon\@$your_domain
To: $supposed_sender_address
Subject: Undeliverable message

The message you sent on $date to $original_recipient could
not be delivered, as that user does not exist.

SMTP Error: 550, 5.1.0 unknown user
Host: $mail_server_dns [$mail_server_ip]
.
};