Question

Subject: Site down, your name servers are down
I am having trouble reaching my website http://example.com/ from different networks. Are your nameservers working
properly?

Answer

Hello,

Thanks for reaching out to Cloudflare support. 
Could you please confirm the actual domain where you are experiencing this issue? example.com is just an ICANN-owned domain reserved for special use.

I do see two domains in your account. One of them (DOMAINA.com) appears to be correctly configured, whereas the other (DOMAINB.com) has a DNSSEC issue at the moment. This prevents it from properly resolving on networks with DNS resolvers which verify the DNSSEC chain. In this case, the DS record needs to be updated with your domain registrar, XXXXXX. Please reach out to them and ask them to set the correct values, as indicated under https://dash.cloudflare.com/?to=/:account/DOMAINB.com/dns/settings.

Could you please confirm that you are having the issue with DOMAINB.com?

Thank you very much,
Alexander

Reasoning

I took the content of the question literally and assumed the customer actually mentioned "example.com", hence the question for the domain. However, given that I have access to his account I could check the two domains and figure out that one would not resolve properly because of the wrong DNSSEC setup. The fact that it seemed to work from some networks also suggested that.

Tools to use: dig, nslookup, and any online DNSSEC verifier