Summary
Forward on a new thread. to is required (400 missing_to). Subject defaults to Fwd: plus the original subject unless it already starts with fwd: or fw:. The body adds a forwarded header block and the original text or html. include_attachments defaults to true and copies the original files. More than 10 files, a file over 4 MiB, or a message over 5 MiB returns 400 attachments_too_large. Allow and block lists are checked; 403 recipient_blocked sends nothing. client_id retries do not send twice. Idempotency-Key works the same way as on send (example: Idempotency-Key: fwd-001).
Request
{
"to": [
"other@example.com"
],
"text": "Sharing this.",
"include_attachments": true,
"client_id": "fwd-001"
}Response
HTTP 201. Sample values illustrate the schema.
{
"id": "inb_example",
"inbox_id": "inb_example",
"thread_id": "thr_example",
"direction": "inbound",
"from": "ada@example.com",
"to": [
"string"
],
"subject": "Hello",
"text": "Tuesday works.",
"html": "string",
"extracted_text": "string",
"extracted_html": "string",
"preview": "string",
"delivery_events": [
{
"type": "string",
"recipient": "string",
"created_at": "2026-10-18T12:00:00.000Z",
"detail": "string"
}
],
"cc": [
"string"
],
"bcc": [
"string"
],
"reply_to": "string",
"in_reply_to": "string",
"references": [
"string"
],
"labels": [
"string"
],
"auth": {
"spf": "string",
"dkim": "string",
"dmarc": "string"
}
}Errors
- HTTP 400 missing_to
- HTTP 400 attachments_too_large
- HTTP 403 recipient_blocked
Idempotency
Idempotency-Key is 1 to 256 characters of letters, digits, and - . _ ~. The same key and the same request within 24 hours returns the original response with HTTP 200 and Idempotent-Replayed: true. A different request is HTTP 409 idempotency_key_conflict. A second call while the first is running is HTTP 409 idempotency_request_in_progress. An empty or invalid key is HTTP 400 invalid_idempotency_key. client_id is checked first when both are set.