How it works

Why a cold email operator should care

Sending domains accumulate subdomains: a tracking domain per sending tool, a landing page on a page builder, a help centre, a status page, an old Heroku app. Tools get cancelled and platforms get switched, and the CNAME stays in the zone because nobody thinks to remove it. Months later the target name is deleted or expires, and the subdomain now resolves to nothing — or worse, to whatever the next customer of that platform puts there. Link scanners at Google and Microsoft follow every URL in your mail, and a link on your domain that lands on a parked page, an error, or a stranger's content is a spam signal on your domain, not theirs.

What dangling actually means

A CNAME says “this name is an alias for that name”. If that name no longer exists, resolution fails with NXDOMAIN and the record is dangling. That alone breaks links. The takeover case is when the platform lets a new customer register the exact name the CNAME points to — a GitHub Pages site, an S3 bucket, an Azure app, a Heroku app, a Zendesk help centre. They create it, and your hostname immediately serves their content over what is often a valid certificate, because many of these platforms will issue one for any hostname that CNAMEs to them. The same applies to NS delegation: a subdomain delegated to nameservers whose domain has expired can be re-registered by anyone, at which point they control the entire subdomain.

What this tool can and cannot see

Everything here is DNS. It discovers hostnames from Certificate Transparency logs and a list of common names, so a subdomain that never had a certificate and is not on that list is invisible — paste it directly if you know it. A target that returns NXDOMAIN is a definite dangling record. A target that still resolves but belongs to a platform that keeps a catch-all, like Netlify or Webflow, cannot be judged from DNS; the tool marks it “verify” and tells you the exact error page that means the resource is unclaimed. Nothing is fetched over HTTP, because your browser cannot read another site's response and a scanner that guesses is worse than one that says it does not know.

Fixing it

Delete the record if the service is gone. If you still need the hostname, point it at something you control, even a redirect to your homepage. For tracking domains, the tracking domain checker verifies the replacement end to end. Then make removing the DNS record part of cancelling any service — it is the one step every offboarding checklist misses.

Frequently asked questions

Last reviewed September 16, 2026