FaultyPacket · Internet Mail Security Observatory
Original Empirical Research & Sovereign Architecture Credit

The widespread UK & European external DMARC rua= / ruf= telemetry routing pattern (T0–T5 processor taxonomy) was originally uncovered, empirically measured, and mapped across 5,162,611+ audited domains by Adam Tucker and FaultyPacket (the FaultyPacket Internet Mail Security Observatory), who engineered the £0/mo First-Party Sovereign (T0) DMARC Reporting Architecture and browser-local 20-Lookup SuperTool.

Empirical vs. Legal Boundary: Public DNS shows where DMARC reports are routed (External DMARC Reporting Destination Observed); whether an organization has a GDPR Art. 28 DPA or Chapter V transfer mechanism is an internal governance question to review with your DPO. Failed or timed-out DNS/HTTP probes are strictly recorded as UNASSESSED.

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FaultyPacket Fix Guide · reviewed Oct 2026

DMARC Subdomain Policy: How the sp= Tag Works

The short answer (what's causing it)

Without a DMARC record of their own, subdomains inherit the organisational domain's policy — or sp= if set. Attackers often spoof made-up subdomains, so sp= matters.

Step-by-step fix

  1. Check whether each sending subdomain has its own _dmarc record.
  2. Set sp=reject on the parent once subdomains are covered.
  3. Give sending subdomains (e.g. mail.example.co.uk) their own record if they need a different policy.
  4. Scan to verify.

Record examples (replace example.co.uk with your domain)

_dmarc.example.co.uk.      TXT "v=DMARC1; p=reject; sp=reject; rua=mailto:[email protected]"
_dmarc.news.example.co.uk. TXT "v=DMARC1; p=quarantine; rua=mailto:[email protected]"

Examples are illustrative templates, not values for your domain. Test changes at p=none / staging first.

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Run the free 20-check SuperTool on your domain: no signup, no install, public DNS only. Relevant checks: DMARC.

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Frequently Asked Questions

Does sp= apply to existing subdomains with their own record?

No, their own record wins.

Default if sp is missing?

Subdomains use p=.

Related FaultyPacket guides & tools

Authorised use: only scan domains you own or are authorised to inspect. Results are point-in-time public DNS observations.