Reason
For the recipient domain specified in the RCPT TO, no valid result was returned to our mail system during MX-DNS resolution.
The domain did not exist at the time of the delivery attempt (NXDOMAIN).
Solution
The person responsible for the domain must ensure that the MX resource record exists for the specified domain.
If the error persists, contact the recipient by other means if necessary.
See also: General information on our error messages.