If I own a domain how do I get reverse DNS to work for mail?

From Christopher Wozniak @TheWozbo via Twitter

If I own a domain on NameCheap, how do I get Reverse DNS to work for mail?  I can't seem to get it to work even though its validated. I have followed aka.ms/d1120 instructions and I get an error: "The reverse DNS FQDN <foo> must resolve to one of: abcd" 

https://twitter.com/TheWozbo/status/633643652506517505

Thanks,
@AzureSupport