What is grDNS?

After sending out an email newsletter using our website we got back several failure notices with the following rather cryptic message:

Connected to ###.###.###.### but greeting failed.
Remote host said: 554 5.7.1 Connection refused. No grDNS allowed

I’m wondering just what this message is supposed to mean. Is there some DNS setting that we failed to set up properly in Cloudflare or is complaining about something else? As I recall most of these failure messages came back from optonline.net.

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