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).