This means that your SMTP server connection is not accepting the email. You would need to look through your SMTP server logs to find out why. Is there a network break to your SMTP server at the time the schedule is sent? It may be a firewall issue too. Is CRD set to "allowed" in your firewall?
Try a different SMTP server and see if that works. If the problem is intermittent then you can switch on "Retry failed mails" in options so that the email is continually retried until the server accepts it or network is back.
