title: How reliable is checkdnsrr (for email domain validation)?
body: <p>I am wondering whether there is a realistic chance of a false negative when utilising checkdnsrr to check the domain part of a provided email address? I want to check this coming from a registration form to reduce the amount of mistypes on the client side.</p>
  
  <pre><code>$hostCheck = checkdnsrr("test.org", "MX");
  </code></pre>
  
  <p>Is checkdnsrr just "pretty reliable" or is there no practical chance of missing an actually existing domain? Because then I would omit such a check. I would appreciate an explanation on the WHYs of the reliability of the whole process. I have researched DNS (some), but I am not certain whether things like servers being temporarily offline or something else might have a measurable impact here.</p>