Without DNS servers, your domain won’t work. Generally we provide 8 DNS servers for each domain to ensure your domain does not go down. While a website might go down, having 8 DNS servers generally assures that your email is working, assuming the service is provided by a different server

OpenSRS

These servers are provided by OpenSRS and are available to any customer who has registered their domain with OpenSRS. Generally despite being only 3 DNS servers have pretty good uptime. Here are the results of their monitoring