This bounce occurs when the recipient’s server is currently unable to deliver your email due to DNS issues on their end. This may or may not be a temporary problem.
The error could be due to the mail server being down, or there was a typo when it was set up, or maybe the destination domain does not exist.
All we know is that the DNS host is unreachable. Therefore we treat this as a soft bounce and allow some time for the problem to be rectified.
To know more about Email Bounces and Error Codes, click here