DMARC treats subdomains more leniently than the domain itself
The DMARC record on `your domain` sets `p=quarantine or reject` for the domain but `sp=a weaker policy` for its subdomains, and sp overrides p for every name below the domain.
Why it matters
The DMARC record on `your domain` sets `p=quarantine or reject` for the domain but `sp=a weaker policy` for its subdomains, and sp overrides p for every name below the domain. Mail forged from `billing.your domain` or `account.your domain` is therefore handled as `a weaker policy` — which, to the person receiving it, is a convincing-looking address your policy is not protecting. A weaker sp is a normal step while you bring a subdomain's own sending into alignment, so confirm this is deliberate; if it is left over from a rollout that finished, remove the `sp=` tag so subdomains inherit `p=quarantine or reject`.
How ShipReady detects it
Email-authentication DNS records (DMARC / SPF). Almost every AI-built SaaS sends transactional email — password resets, magic links, receipts — through Resend, SendGrid or Postmark. Setting that up gets the mail delivered; it does not stop anyone else sending mail that claims to come from the same domain. Without a DMARC policy, a receiving server has no instruction to reject a forgery, so an attacker can send "reset your password" from the product's own domain and the message will land in the inbox looking authentic. This check is almost entirely DNS rather than HTTP — one exception is MTA-STS, whose policy is only half-published in DNS; the other half is a file fetched over HTTPS (see _check_mta_sts). Nearly everything else here is the most deterministic kind of check the scanner has: a record either exists or it does not, so there is no heuristic and no false-positive surface. The one place judgement is required is deciding WHICH domain to ask about — see _organizational_domain.
Detection is deterministic. ShipReady reports this only when it observes the condition directly, and prefers to miss a real problem over inventing one. Rule version 1.10.0.
How to fix it
This is the prompt ShipReady puts in your report — written to be pasted straight into Cursor, Claude Code, or whichever assistant built the app.
The DMARC record on your domain sets p=quarantine or reject for the domain but sp=a weaker policy for its subdomains, and sp overrides p for every name below the domain — so mail forged from billing.your domain or account.your domain is handled at the weaker setting. If the weaker sp was a temporary step while a subdomain's own sending was brought into alignment and that work is finished, remove the sp= tag entirely: subdomains then inherit p=quarantine or reject, which is what you want. If a subdomain still needs a weaker policy, publish a DMARC record on that specific subdomain instead of weakening every subdomain at once. If you send no mail from subdomains at all, the strictest configuration is sp=reject plus a null MX record on any subdomain that receives none.
Frequently asked questions
- What does "DMARC treats subdomains more leniently than the domain itself" mean?
- The DMARC record on `your domain` sets `p=quarantine or reject` for the domain but `sp=a weaker policy` for its subdomains, and sp overrides p for every name below the domain.
- How serious is it?
- ShipReady rates this low. Worth fixing. Small individually, and they accumulate.
- How do I fix it?
- Paste the fix prompt on this page into Cursor, Claude Code or your AI editor. It is the same prompt ShipReady puts in your report.
- Can I check my own site?
- Yes — ShipReady scans up to ten pages of any public site for free and reports this alongside every other check. The free report lists every issue it finds and shows full evidence and a fix prompt for the critical and high-severity ones; medium and low findings are counted and unlock on Pro.
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