# POST /inboxes/{inbox_id}/threads/{thread_id}/emails/{email_id}/reply (Resend API)
Base URL: https://api.resend.com
Auth: Authorization: Bearer <RESEND_API_KEY>
Last verified: 2026-10-20

> Reply to a thread email

## Critical Gotchas
* All requests must include a `User-Agent` header (e.g. `User-Agent: my-app/1.0`) or the API returns HTTP 403.

## cURL
curl -X POST 'https://api.resend.com/inboxes/inbox-id/threads/thread-id/emails/email-id/reply' \
  -H "Authorization: Bearer $RESEND_API_KEY" \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{"html":"<p>Refund issued for order 1041.</p>","cc":["billing@acme.com"]}'

## Parameters
| Name | In | Type | Required | Description |
| :--- | :--- | :--- | :--- | :--- |
| inbox_id | path | string | Yes | The Inbox ID or the inbox email addre... |
| thread_id | path | string | Yes | The Thread ID. |
| email_id | path | string | Yes | The Email ID of the message to reply... |
Other params: cc, bcc, html, text, subject, reply_all, attachments, headers

## Response (200 OK)
{"id":"6a0c8e58-3d9e-4f7a-8b14-2e3f4a5b6c71"}

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