So you created a proxy to an endpoint to an email from iCloud and something in the chain had a misconfigured DNS with the domain “p00-icloudmta-asmtp-us-central-1k-100-percent-10.p00-icloudmta-asmtp-vip.icloud-mail-production.svc.kube.us-central-1k”. It might as well be an issue with mail-testers.com or icloud.com

It’s impossible to tell from the shared page because both services are about DNS caching.