We could not resolve the DNS hostname you provided to an IP address to attempt a connection. This is a critical problem.
This is only possible if you specified a hostname to test against. If you know the IP address of your mail server you can run the test using the IP to bypass this error, however doing so will mask a real problem with your DNS.
You should perform a DNS Lookup to ensure you have an A record mapping your hostname to the correct IP address for your server. You may also use our DNS Check tool to make sure your DNS servers are up and running properly.