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Tracking domain CNAME checker

Follow a tracking subdomain’s CNAME chain and see whether it ends at a known ESP’s tracking host.

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Result

The verdict appears here once you press Check the tracking domain.

What this is

This custom tracking domain checker follows a tracking subdomain’s CNAME chain, hop by hop, and says whether it ends at one of the ESP tracking hosts this page cites by name - or at an address record, which may simply mean the ESP sending it is not yet on that cited list.

A custom tracking domain lets click and open tracking links carry your own domain instead of your email service provider’s shared one. Setting it up is a CNAME record: your tracking subdomain points at a host the ESP tells you to alias it to, and this tool follows that alias to see where it actually leads.

Several ESPs publish a fixed hostname their tracking CNAME always resolves to, which is what this tool matches against - a short, cited list, kept to the vendors whose own documentation states a fixed target rather than a guess from a third-party summary.

How to read the result

The tree shows every CNAME hop in order, ending either at a name this tool recognises as a cited ESP tracking host, or at a name it does not recognise. A name it does not recognise is not automatically wrong - it may be an ESP that is not on this cited list yet.

The findings above the tree point out anything else worth knowing: a CNAME chain several hops deep, a name with no CNAME at all, a chain this tool gave up following after 5 hops, or a tracking link on the sending domain itself with no subdomain.

One of our guides, "What Google and Yahoo require from bulk senders, and how to prove you meet it," covers this alongside the rest of Google's and Yahoo's own bulk-sender rules, and the parts no tool, including this one, can check.

Common mistakes

What goes wrong most often, and how to tell.

  • Putting the tracking link on the root sending domainMailchimp Transactional’s own docs require a subdomain for a tracking CNAME rather than the root domain. A root-domain entry is flagged here, with the same point every ESP’s setup instructions make: use a dedicated subdomain.
  • Assuming an unmatched host means the setup is brokenThis tool’s list is short on purpose - only the ESPs whose own documentation states a fixed CNAME target are on it. A target that matches none of them may simply be an ESP that has not been confirmed yet, not a fault in your DNS.
  • Treating a matching CNAME as proof tracking is liveA matching CNAME only confirms the DNS points at the right host. The ESP still has to have the custom tracking domain configured and verified on its own side, which this tool cannot see at all.

Questions

What people ask about this tool.

  • It is not automatically a fault. This tool’s list is kept to the ESPs whose own documentation states a fixed CNAME target, confirmed directly at the vendor’s docs. An unmatched target may simply be an ESP that is not on that cited list yet.

  • It confirms the DNS points at the right host, which is the half this tool can see. It cannot confirm the ESP has the custom domain configured and verified on their own side - that only shows up inside the ESP’s own dashboard.

  • Mailchimp Transactional’s own documentation requires a subdomain rather than the root domain for a tracking CNAME, and vendors such as SparkPost/Bird separately give subdomain isolation as a general reason to keep a sending-related domain apart from the rest of your main domain.

  • Five hops is already far more than a normal tracking-domain setup needs. A chain still pointing at another CNAME after five hops is unusual enough that this tool stops and says so, rather than following it indefinitely.

Sources

Every fixed value here comes from the provider’s own documentation. These pages change, so each says the day it was last read.

Use it from the API

The same check, as one request.

Request
curl -X POST https://resellerhq.com/api/tools/tracking-domain-cname-checker \
  -H "Content-Type: application/json" \
  -d '{"subdomain":"example.com"}'
Response
{
  "tool": "tracking-domain-cname-checker",
  "input": {
    "subdomain": "example.com"
  },
  "result": {
    "verdict": {
      "tone": "success",
      "headline": "example.com points at SendGrid's tracking host",
      "summary": "The CNAME chain ends at sendgrid.net, which matches SendGrid's cited tracking host."
    },
    "findings": [
      {
        "id": "no-subdomain-entered",
        "severity": "warn",
        "title": "This is the root domain, with no subdomain at all",
        "detail": "example.com has no subdomain in front of it - it is the sending domain itself. A tracking link needs its own subdomain so it can be isolated from (and retired separately from) the rest of the domain.",
        "fix": "Create a dedicated subdomain such as links.example.com and CNAME that to your ESP's tracking host instead of using the root domain directly."
      },
      {
        "id": "subdomain-isolates-reputation",
        "severity": "info",
        "title": "ESP docs put tracking links on a subdomain, not the root domain",
        "detail": "Mailchimp Transactional's own docs require a subdomain for a tracking CNAME, not the root domain: \"You must use a subdomain (like mail.example.com) and not a root domain (like example.com)\" (Mailchimp, read 2026-10-13). More generally, SparkPost/Bird's own sending-domain guidance gives the reason for subdomain isolation: \"It keeps your sending reputation separate from everything else on the domain\" (SparkPost/Bird, read 2026-10-13). No vendor documentation states the stronger claim that a tracking link on the root domain itself harms reputation - only that a subdomain is the documented requirement and general isolation practice."
      },
      {
        "id": "matches-known-esp",
        "severity": "pass",
        "title": "Matches SendGrid's tracking host",
        "detail": "sendgrid.net is on the cited list of ESP tracking hosts for SendGrid, confirmed at SendGrid's own documentation."
      }
    ],
    "parts": [
      {
        "kind": "tree",
        "title": "CNAME chain",
        "nodes": [
          {
            "label": "example.com",
            "children": [
              {
                "label": "sendgrid.net",
                "note": "matches SendGrid's tracking host",
                "tone": "success"
              }
            ],
            "note": "CNAME"
          }
        ]
      }
    ],
    "lookups": [
      {
        "label": "CNAME chain for example.com",
        "value": "1 hop, ending at sendgrid.net"
      }
    ],
    "limits": [
      "This confirms the DNS points at the ESP's tracking host. It cannot confirm the ESP has this custom domain configured and verified on their own side - that is only visible inside the ESP's own dashboard."
    ]
  }
}

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