Reply to Thread Email

Send a reply to a message in an inbox thread.

Inboxes are currently in private beta and only available to a limited number of users. The response shape might change before GA.

Get early access if you're interested in testing this feature.

Once you have access, upgrade your Resend SDK to use the new methods:

Node.js

npm install resend@6.28.1-preview-inboxes.2

CLI

npm install -g resend-cli@2.22.0-preview-inboxes.2

Replies from the inbox address. Recipients are taken from the message you reply to.

At least one of html or text is required.

Path Parameters

The Inbox ID.

The Thread ID.

The Email ID of the message to reply to, as returned in the thread's messages[].id.

Body Parameters

html (string) — The HTML body.
text (string) — The plain-text body.
subject (string) — The subject of the reply. When omitted, the thread subject is used, prefixed with Re: if it isn't already.

Response Fields

id (string) — The ID of the queued outbound email.
email_id (string) — The ID of the queued outbound email. Same value as id.
direction (string) — Always outbound.
from (string) — Sender email address.
to (string[]) — The recipients of the message.
cc (string[]) — The CC recipients of the message.
bcc (string[]) — The BCC recipients of the message.
reply_to (string[]) — Always an empty array.
subject (string) — The subject of the sent reply.
message_id (string | null) — Always null.
html (string | null) — The HTML body.
text (string | null) — The plain-text body.
attachments (array) — The attachments on the message.
properties
- id (string) — The ID of the attachment.
- filename (string | null) — The filename of the attachment.
- size (number | null) — The size of the attachment in bytes.
read (boolean) — Whether the message has been read.
received_at (string) — ISO 8601 timestamp when the message was sent.

Node.js

import { Resend } from 'resend';

const resend = new Resend('re_xxxxxxxxx');

const { data, error } = await resend.inboxes.threads.emails.reply({
  inboxId: 'b3e2b2b6-3f0e-4c8e-9ad3-2f43a1e2c7f1',
  threadId: '4d8e2a1c-9b3f-4c6d-8a21-3e5f7c9c0d12',
  emailId: '5b1a9f47-2c8d-4e6f-9a03-1d2e3f4a5b60',
  html: '<p>Refund issued for order 1041.</p>',
  text: 'Refund issued for order 1041.',
});

cURL

curl -X POST 'https://api.resend.com/inboxes/b3e2b2b6-3f0e-4c8e-9ad3-2f43a1e2c7f1/threads/4d8e2a1c-9b3f-4c6d-8a21-3e5f7c9c0d12/emails/5b1a9f47-2c8d-4e6f-9a03-1d2e3f4a5b60/reply' \
     -H 'Authorization: Bearer re_xxxxxxxxx' \
     -H 'Content-Type: application/json' \
     -d $'{
  "html": "<p>Refund issued for order 1041.</p>",
  "text": "Refund issued for order 1041."
}'

CLI

resend inboxes threads emails reply \
  --inbox_id b3e2b2b6-3f0e-4c8e-9ad3-2f43a1e2c7f1 \
  --thread_id 4d8e2a1c-9b3f-4c6d-8a21-3e5f7c9c0d12 \
  --email_id 5b1a9f47-2c8d-4e6f-9a03-1d2e3f4a5b60 \
  --html "<p>Refund issued for order 1041.</p>" \
  --text "Refund issued for order 1041."

Response

{
  "id": "6a0c8e58-3d9e-4f7a-8b14-2e3f4a5b6c71",
  "email_id": "6a0c8e58-3d9e-4f7a-8b14-2e3f4a5b6c71",
  "direction": "outbound",
  "from": "support@example.com",
  "to": ["Ada Lovelace <ada@example.org>"],
  "cc": [],
  "bcc": [],
  "reply_to": [],
  "subject": "Re: Refund for order 1041",
  "message_id": null,
  "html": "<p>Refund issued for order 1041.</p>",
  "text": "Refund issued for order 1041.",
  "attachments": [],
  "read": true,
  "received_at": "2026-08-05T14:12:04.110Z"
}