curl -X POST 'https://api.resend.com/inboxes/inbox-id/threads/thread-id/emails/email-id/forward' \
  -H "Authorization: Bearer $RESEND_API_KEY" \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{
  "to": [
    "colleague@example.org"
  ],
  "cc": [
    "billing@acme.com"
  ],
  "bcc": [
    "records@acme.com"
  ],
  "html": "<p>Flagging this refund request for you.</p>",
  "text": "Flagging this refund request for you.",
  "subject": "Fwd: Refund for order 1041",
  "attachments": [
    {
      "content": "example-content",
      "filename": "example-filename",
      "path": "example-path",
      "content_type": "example-content-type",
      "content_id": "example-content-id"
    }
  ],
  "headers": {
    "X-Ticket-Id": "4821"
  }
}'

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

> Forward 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.
* The `to` field accepts either a single recipient email address string or an array of strings for multiple recipients (max 50).

## cURL
curl -X POST 'https://api.resend.com/inboxes/inbox-id/threads/thread-id/emails/email-id/forward' \
  -H "Authorization: Bearer $RESEND_API_KEY" \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{"to":["colleague@example.org"]}'

## Parameters
| Name | In | Type | Required | Description |
| :--- | :--- | :--- | :--- | :--- |
| inbox_id | path | string | Yes | The Inbox ID or the inbox e... |
| thread_id | path | string | Yes | The Thread ID. |
+9 more params

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

---
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POST

POST /inboxes/{inbox_id}/threads/{thread_id}/emails/{email_id}/forward — Resend API

Resend API

Forward a thread email

Base URL: https://api.resend.com
Auth: Authorization: Bearer <RESEND_API_KEY>
Last verified: 2026-10-20 · upstream hash matched

Actions Try in browser→

Agents: curl -H "Accept: text/markdown" this URL
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Critical gotchas

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

The to field accepts either a single recipient email address string or an array of strings for multiple recipients (max 50).

cURL

curl -X POST 'https://api.resend.com/inboxes/inbox-id/threads/thread-id/emails/email-id/forward' \  -H "Authorization: Bearer $RESEND_API_KEY" \  -H 'User-Agent: my-app/1.0' \  -H 'Content-Type: application/json' \  -d '{  "to": [    "[email protected]"  ],  "cc": [    "[email protected]"  ],  "bcc": [    "[email protected]"  ],  "html": "<p>Flagging this refund request for you.</p>",  "text": "Flagging this refund request for you.",  "subject": "Fwd: Refund for order 1041",  "attachments": [    {      "content": "example-content",      "filename": "example-filename",      "path": "example-path",      "content_type": "example-content-type",      "content_id": "example-content-id"    }  ],  "headers": {    "X-Ticket-Id": "4821"  }}'

Parameters

Name	In	Type	Required	Description
inbox_id	path	string	Yes	The Inbox ID or the inbox email address, such as `[email protected]`. A value that contains `@` must be a valid email address, and any other value must be a UUID, or the request returns a 422. The address match is case-insensitive and never falls back to the inbox `name`. An address that matches no inbox returns a 404.
thread_id	path	string	Yes	The Thread ID.
email_id	path	string	Yes	The Email ID of the message to forward, as returned in `data[].id` by the list thread emails endpoint.
to	body	string	Yes	Recipient email address. For multiple addresses, send as an array of strings. Must name at least one address. `to`, `cc` and `bcc` combined cannot exceed 50 recipients. Each address is `[email protected]` or `Name <[email protected]>`. An address that is not valid returns a 422.
cc	body	string	No	CC recipient email address. For multiple addresses, send as an array of strings. Counts toward the 50-recipient limit. Each address is `[email protected]` or `Name <[email protected]>`. An address that is not valid returns a 422.
bcc	body	string	No	BCC recipient email address. For multiple addresses, send as an array of strings. Counts toward the 50-recipient limit. Each address is `[email protected]` or `Name <[email protected]>`. An address that is not valid returns a 422.
html	body	string	No	An HTML note sent above the quoted original.
text	body	string	No	A plain-text note sent above the quoted original.
subject	body	string	No	The subject of the forward. Cannot be only whitespace or contain a line break. When omitted, the thread subject is used, prefixed with `Fwd:` if it does not already start with it.
attachments	body	object[]	No	Files to attach, as on `POST /emails`. Each one needs `content` or `path`. They share one limit with the attachments of the original message, which the forward also sends. Together they can have at most 25 MB of `content`. A `path` file does not count toward the 25 MB, because it is fetched when the email is sent. An empty file, a blocked file type, a `path` that does not start with `http` or that starts with `http://localhost:`, or a `content_type` that is not a valid MIME type returns a 422. Other `path` problems fail the email when it is sent, not the request.
headers	body	object	No	Custom headers to add to the email. The `In-Reply-To`, `References` and `Reply-To` headers are set by Resend and cannot be set here. Sending one of them, in any letter case, returns a 422. Header names cannot be empty, and names and values cannot contain line breaks or null characters, or the request returns a 422. `null` means no custom headers.

Response 200 OK

{
  "id": "6a0c8e58-3d9e-4f7a-8b14-2e3f4a5b6c71"
}

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