DNS (Domain name service) is the thing that translates a name like google.com into an IP address like 220.127.116.11. rDNS (reverse Domain name service) is the thing that does the translation the other way round. You need DNS so that people can use names like google.com instead of having to remember a stream of numbers. Reverse DNS, as far as I can see, is only used by some mail systems to prevent mail from being delivered if it came from a server that doesn't have rDNS. AOL, for example, do this, and very few others, because I don't think it has much effect on the flow of spam.
So, today, an email I sent bounced, and when I looked at the bounce, it wasn't the usual "email address doesn't exist", it was "no reverse DNS". What? WHAT???
I have reverse DNS, I know I do, I went to some trouble to get it working and make sure it worked. But suddenly I didn't. I tried "dig +trace 18.104.22.168.in-addr.arpa ptr", but using the actual email address of the server, and it went into an infinite loop. What that is supposed to do, is display the chain of servers that try to do the rdns, and pass it on to another server that's delegated to do it.
And that meant that it's a Daisy issue. Daisy are supposed to delegate the rDNS for the block of IP addresses that I use to one of my servers, so that I can do the actual rDNS. That delegation clearly was no longer happening. Daisy supplies my leased line, and I'm not terribly surprised when they cock something up. So I phoned Daisy, got transferred to someone who knew about DNS and rDNS, explained the problem, and sent them an email giving details, and name servers, and explaining the problem. Then I went out caching, because there's not much else I can do.
I'm pleased to report that Daisy fixed whatever it was they'd cocked up, so I have my rDNS working again.