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.
Are you managing your email delivery?
Email delivery is more than running an email server, or regularly checking for blacklists. To really ensure you customers get your email, you need to setup SPF, DKIM and DMARC. If you aren't manage your DMARC configuration and responses from email providers, you don't know if your customers are getting your email.
MxToolbox is YOUR expert on email deliverability. MxDelivery Center can help you setup and analyze your DMARC, DKIM and SPF to give you the insight you need to make email configuration changes and get your emails to your customer's.