List inboxes
Returns all inboxes registered to your account, including both platform-domain and custom-domain inboxes.
**Response fields (per inbox):**
- `inbox` — Full email address of the inbox
- `domain` — Domain portion of the address
- `createdAt` — ISO 8601 timestamp of when the inbox was registered
- `expiresAt` — Expiry timestamp (based on your plan's persistence tier)
- `messageCount` — Number of messages currently stored
**Behaviour:**
- Returns an empty array `[]` if no inboxes are registered.
- Inboxes are returned in descending order of creation time (newest first).
**Plan:** All plans (Free and above).