Add IntoDNS mail and DNS server checker.

README.md

@@ -273,6 +273,8 @@ Before add pull request please see **[this](https://github.com/trimstray/the-boo
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://dnstable.com/"><b>DNS Table online</b></a> - search for DNS records by domain, IP, CIDR, ISP.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="http://ptrarchive.com/"><b>PTRarchive.com</b></a> - this site is responsible for the safekeeping of historical reverse DNS records.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="http://xip.io/"><b>xip.ip</b></a> - wildcard DNS for everyone.<br>
+&nbsp;&nbsp;:small_orange_diamond: <a href="https://intodns.com"><b>INTO DNS</b></a> - DNS and mail server health checker.<br>
+
 </p>
 
 ##### :black_small_square: Mail