Multiple SERVFAIL/REFUSED unexpected RCODE

[Mik J]

Hello,
In my named.log I can see a lot of SERVFAIL/REFUSED unexpected RCODE messages. Most of the time someone tries to resolve a PTR
I can see an average of 10 messages per second like these
May  3 10:46:26 dns named[7228]: REFUSED unexpected RCODE resolving 'x.204.99.116.in-addr.arpa/PTR/IN': 203.113.131.x#53
May  3 10:46:26 dns named[7228]: SERVFAIL unexpected RCODE resolving 'x.16.165.88.in-addr.arpa/PTR/IN': 193.0.9.x#53

The PTR records don't belong to me and the remote DNS servers are located around the world.
Does anyone has an understanding of why I receive these type of requests ? Why do they query my DNS servers ?
Thank you
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