The email could not be delivered because the sender domain is not configured in accordance with RFC.
We expect an MX record which itself has a valid A record.
Please check your DNS records in this regard.
For more information, see the RFCs at the following links:
If you have any questions or require further support, please contact the administrator of your domain.
Please note that in the case of inquiries to our postmaster team, a reply to your domain will not be possible as long as the misconfiguration persists. In these cases, please provide an alternative contact email address of another domain.
See also: General information on our error messages.