Reason
For the recipient domain specified in the RCPT TO, no valid result was returned to our mail system during MX-DNS resolution.
This is caused by DNS timeouts or SERVFAILs, i.e. the domain is available, but the responsible name server is not currently answering any queries about the domain.
Solution
It can be assumed that your email system will automatically attempt to deliver the email at a later time.
This process is usually repeated several times over a certain period of time.
If the condition persists for longer, contact the recipient by other means if necessary.
See also: General information on our error messages.