Messages that don't arrive (poor deliverability) or that arrive without consent (TCPA/ CAN-SPAM violations) are both fatal for an SMB product. This skill makes both correct by construction. Design the consent + deliverability posture before the first send.
1. Transactional vs marketing — split them
Decide per message which bucket it is; they have different rules and should use different sending identities (often different subdomains / providers):
| Transactional | Marketing / lifecycle | |
|---|---|---|
| Examples | receipt, booking confirm/reminder, password reset | win-back, promo, newsletter, nurture step |
| Consent | implied by the transaction | explicit opt-in required |
| Unsubscribe | not required (but honor STOP) | required, one-click, honored fast |
| Sending domain | txn. subdomain |
mail./news. subdomain |
Never send marketing content on the transactional channel "because it delivers better" — that's how the transactional domain gets burned.
2. Provider selection (pick one, justify it)
- Email — Postmark (best transactional deliverability, strict on marketing), Resend (DX-first, good default), SendGrid (scale). Default: Resend for transactional, add a marketing-grade ESP only when lifecycle volume justifies it.
- SMS — Twilio (messaging service + sender pool), or Telnyx. Use a Messaging Service, not a single number, for scale + failover. A2P 10DLC registration is required for US application-to-person SMS — register the brand/campaign before sending.
3. Domain authentication (non-negotiable for email)
- SPF — sender IP authorized in DNS.
- DKIM — provider signing key published; messages signed.
- DMARC — start
p=nonewith rua reporting, ramp top=quarantine→p=rejectonce aligned. Without DMARC alignment, lifecycle mail lands in spam. - Warm up a new sending domain gradually; never blast from a cold domain.
4. Consent + compliance (US-first)
- CAN-SPAM (email) — valid physical postal address, accurate From/Subject, working one-click unsubscribe honored within 10 days.
- TCPA (SMS/voice) — prior express written consent for marketing SMS; honor STOP/UNSTOP/ HELP keywords automatically; respect quiet hours (no marketing 9pm–8am recipient local time). Keep proof of consent (timestamp, source).
- CASL (if CA recipients) — express opt-in + identification + unsubscribe.
- Double opt-in for marketing lists where feasible — protects deliverability and proves consent.
5. Suppression discipline (the deliverability lifeline)
Maintain a single suppression list the sender checks before every send:
- hard bounces → suppress permanently
- spam complaints (FBL) → suppress + investigate
- unsubscribes / STOP → suppress for that channel immediately
- never re-import a suppressed address from a migration without re-consent
A send that ignores suppression is the fastest path to a blocklist.
6. Reliability patterns
- Idempotent sends keyed on the domain event (a reminder for booking X sends once, even on retry) — coordinate the key with integrations-engineer.
- Status-callback / webhook reconciliation: record delivered/bounced/failed; surface failures, don't swallow them.
- Rate-limit + queue lifecycle sends; never loop-send.
- Quiet-hours + timezone are computed from the recipient's locale, not the server's.
Output
When applied, contribute a Messaging section to the integration contract
(docs/integrations/INTEGRATE-{slug}.md):
## Messaging
- channels: email <provider> / sms <provider>
- identities: txn = <subdomain>, marketing = <subdomain/ESP>
- domain auth: SPF/DKIM/DMARC plan = <state>
- consent: <implied/explicit per message type>; STOP/HELP = handled
- quiet hours: <recipient-local window>; 10DLC: <registered?>
- suppression: <store> checked pre-send
- idempotency key: <derivation>