Reason
The email could not be delivered because a temporary error occurred during the Domain Name System (DNS) query to determine whether a domain name is assigned to the IP address of the email server you are using (query the PTR RR).
The prerequisite for accepting an email is that the reverse DNS information of the mail server IP address is available.
Solution
Your email system should automatically attempt to deliver the email again at a later time due to the error code 4xx.The problem may no longer occur at this time.
We therefore ask for your patience.
If the problem persists, please check the availability of the DNS infrastructure you are using.
See also: General information on our error messages.