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Supersedes 0035. The SPF, DKIM and DMARC steps of 0035 (decisions 1 to 3) carry forward.

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Context

0035 (Proposed) chose Resend with the mail-authentication records. The owner’s answer PP9 (mem L562, 2026-10-20) adds subdomains per environment, separate streams, a DMARC ramp, custom sending domains for tenants, and an SMTP relay on-prem.

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Decision

  1. Subdomains. Email is sent from a subdomain per environment.
  2. Streams. Transactional and marketing mail use separate streams.
  3. DMARC ramp. DMARC policy ramps to reject.
  4. Tenant sending domains. Tenants can use their own sending domain. This is a Pro entitlement (ADR 0242).
  5. On-prem. An SMTP relay is used, behind the same mail driver.
  6. Provider. Resend sits behind the mail driver, so the provider can change.

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Consequences

  • A marketing problem cannot damage transactional delivery, because the streams are separate.
  • A tenant sending domain is a plan feature, not a code path.

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Enforcement

planned — the DNS records are checked by the infrastructure stack (ADR 0171); the mail driver’s contract test covers the SMTP relay.

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Alternatives

  • One stream for all mail: rejected; a marketing complaint would affect password resets.

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Supersedes and sources

Supersedes: 0035. Sources: owner answer PP9 (stackra-decisions-2026-10-19.md L562, 2026-10-20). Closes gap C39.