Summary
Reply on the same thread. Inbound mail goes to Reply-To or From. Mail you sent goes to the original To. Subject is Re: unless it already starts with re:. In-Reply-To and References use the original Message-ID (the last 20 ids). One of text or html is required (400 missing_body). reply_all adds the other recipients, merges cc, drops this inbox, and dedupes (400 missing_to if nobody remains). Allow and block lists are checked; 403 recipient_blocked sends nothing. client_id retries return the original message with idempotent true and do not send again. Idempotency-Key works the same way as on send (example: Idempotency-Key: reply-001). A replay returns HTTP 200 with idempotent true and Idempotent-Replayed: true. The message id in the path is part of the request, so the same key on a different message is 409 idempotency_key_conflict. Unknown messages return 404 message_not_found. A paused inbox returns 403 inbox_paused.
Request
{
"text": "Tuesday still works.",
"cc": [
"copy@example.com"
],
"client_id": "reply-001"
}Response
HTTP 200. 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_body
- HTTP 400 missing_to
- HTTP 403 recipient_blocked
- HTTP 409 idempotency_key_conflict
- HTTP 404 message_not_found
- HTTP 403 inbox_paused
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.