# Notification Delivery Failures

## Objective

Know when a message failed to arrive and stop sending to addresses that cannot receive it.

Bounce and complaint handling with a suppression list, plus telling the affected user their address is failing.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Consume the delivery provider's webhooks for bounces, complaints, and deferrals, verify their signatures, and deduplicate repeated events by provider event ID.
2. Separate hard bounces and spam complaints, which suppress the address permanently, from transient failures, which retry with backoff and only suppress after a threshold.
3. Keep the suppression list per channel and per address, and check it before every send. Extend the existing notification preference model with delivery health rather than building a second gate beside it.
4. Tell the user in the app when their address is suppressed, and give them a way to correct or reverify it. An invisible suppression looks exactly like a broken product.
5. Do NOT let a suppressed email address block push, in-app, or SMS delivery to the same user. Suppression is per channel, never per person.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- A spam complaint must suppress marketing and product mail without stopping password resets, and the boundary needs deciding before launch.
- Provider webhooks arrive out of order. A delivery event landing after a bounce for the same message must not clear the suppression.
- A user correcting their address must clear the suppression for the new address only, and require reverification before mail resumes.
- Admin views must show status and reason without dumping raw provider payloads, which contain recipient data and provider secrets.
- A provider-wide outage will look like thousands of individual failures — detect the pattern and avoid mass-suppressing valid addresses.
- Dead push subscriptions belong on the same failure pipeline as bounced email, not in a separate ad hoc cleanup.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Provider webhooks are signature-verified and deduplicated by event ID.
- [ ] Hard bounces and complaints suppress permanently; transient failures retry with backoff.
- [ ] The suppression list is checked before every send and is scoped per channel.
- [ ] Suppression on one channel does not affect the others.
- [ ] Affected users are told in-app and can correct and reverify their address.
- [ ] Admin views show delivery status and reason without exposing raw provider payloads.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.