The Dormant List Index: Methodology, Definitions and Publication Rules

Updated 1 November 2026. This page defines what the index measures and how. Data editions are published quarterly once the minimum sample in Section 4 is reached; each edition links back here.

1. What the index measures

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Opens are not used in any metric. Apple Mail's privacy proxy records opens no person made (49–62% of recorded opens in 2026 estimates), so an index built on opens would measure device mix.

2. Breakdowns

Each edition reports every metric by:

• List age: time since the oldest unmailed segment was last mailed (under 90 days; 90–365; 1–2 years; over 2 years).
• Acquisition source: signup form; checkout or account creation; import; partner or event; unknown.
• Platform: Klaviyo, Mailchimp, HubSpot, beehiiv, Substack, other.
• Vertical: e-commerce; newsletter or media; B2B SaaS; services; non-profit; other.
• List size band: under 10,000; 10,000–50,000; 50,000–250,000; over 250,000.

Cells with fewer than the minimum sample (Section 4) are not published.

3. How the data is collected

1. Audits. Every List Health Audit produces a snapshot: eligibility holds, scan results, dormancy map by tier, domain risk score. The anonymized aggregates (shares, not addresses) are stored with the breakdown attributes above.
2. Revival cohorts. For clients who run a Revival Sprint, the frozen cohort, its baseline, the daily stop-condition log and the 45- and 90-day read-outs are recorded in the same way.
3. Anonymization. No email addresses, domains, client names or campaign content leave the audit system. The index stores percentages and counts per snapshot with the categorical attributes only.
4. Consent. Use of anonymized, aggregated audit results in published research is part of the audit terms. Clients can opt out; opted-out snapshots are excluded.
5. Exclusions. Lists flagged as purchased or scraped are excluded from dormancy and reactivation metrics (they are reported only as a count of excluded snapshots), because their numbers describe acquisition fraud, not list ageing.

4. Publication rules

• Minimum sample. A metric is published only when it rests on at least 20 snapshots (audits) or 10 cohorts (reactivation metrics), and a breakdown cell only when it has at least 8 snapshots or 5 cohorts.
• Statistics. Median and interquartile range for every cell; the number of snapshots or cohorts behind the cell; the total contacts represented.
• Cadence. Quarterly, within six weeks of quarter end. Edition pages are dated and never edited after publication except for corrections, which are logged.
• Stability. Definitions in Section 1 and breakdowns in Section 2 change only with a dated note on this page and a restatement of the previous edition's affected cells.
• Comparability warning. Index figures describe lists that were submitted for an audit, which skews toward lists with a known problem. Each edition states this. The index is a benchmark for "lists like yours when you suspect a problem," not for the population of all email lists.

5. How to cite

Cite the edition page, not this methodology page, for any number, with the quarter and the cell's sample size. Suggested form: EngageMKTG, Dormant List Index Q4 2026, dormant share median for newsletter lists aged 1–2 years (n = 14 snapshots), published [date], [URL]. Each edition carries a "Cite this page" block.

6. What the index will not do

• It will not name or rank clients, platforms' deliverability, or vendors.
• It will not report means, because a few very large or very old lists would dominate them.
• It will not publish a cell that fails the minimum sample, even if the number is interesting.
• It will not claim reactivation results beyond the 45-day click-or-order rate and the 90-day attributed revenue on frozen cohorts, measured the same way for every cohort.
• It will not use opens.

7. Reference points the index replaces

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FAQ

What is the Dormant List Index? A quarterly set of medians describing dormant, invalid and trap-patterned shares and controlled reactivation rates in real email lists, computed from anonymized List Health Audits and revival cohorts.

Where does the data come from? From audits and revival cohorts run on EngageMKTG's tooling, aggregated as percentages with categorical attributes, under consent in the audit terms. No addresses, domains or clients are identifiable.

Why medians rather than averages? List distributions are skewed; a handful of very large or very old lists would dominate an average. Medians with interquartile ranges describe the typical list.

Is the index representative of all email lists? No, and each edition says so. It describes lists that were submitted for an audit, which skews toward lists with a suspected problem.

How can my list be included? Run the free List Health Audit. Anonymized, aggregated results contribute to the next edition unless you opt out.

Contribute a snapshot and get the audit. The free List Health Audit returns your dormancy map, scan results and domain risk score within 72 hours; the anonymized aggregates feed the next edition. Get the free audit →

Sources

• EngageMKTG, Dormant Email List Reactivation Playbook (tier definitions, stop conditions, cohort measurement), Email Sunset Policy (inactivity windows), Spam Traps and Dormant Lists (what a scan can and cannot catch), Newsletter Open Rate Benchmarks 2027 (public benchmark sources and their limits).
• Return Path, win-back study of 33 retailers (2014), as reported by Validity, MarTech and MarketingProfs.
• Inbox Collective, Not a Newsletter (April 2023).
• EmailListValidation (vendor, 2026): 15–30% invalid or risky addresses in dormant lists.
• Stripo and Litmus (2026): Apple share of recorded opens. Secondary.