anondevuser.xyz is hosted on Render, which has no IPv6 It points at budget-iati.onrender.com, and that name has no AAAA record — so nothing you add in your own DNS can give it IPv6. Put an IPv6-capable proxy in front (Cloudflare's proxied DNS does this), or host somewhere that offers IPv6. Which platforms offer IPv6
Mail servers without IPv6 eforward1.registrar-servers.com, eforward2.registrar-servers.com, eforward3.registrar-servers.com, eforward4.registrar-servers.com, eforward5.registrar-servers.com have no AAAA record, so mail from IPv6-only servers can't reach them directly. If a provider runs your mail, that's their setting; if you run it, add AAAA records for these hosts.
Visitors on IPv6-only networks (many mobile carriers) can still reach it, but only through a translator their carrier runs (NAT64). The site itself has no IPv6 address — and you can't make one up from the IPv4. Why some hosts don't offer it
Copy the evidence— a citation-ready record of this measurement