Reply-all to a message in an AgentMail inbox via the MPP-gated email API for AI agents.
What it does
This endpoint is part of the AgentMail MPP gateway, which provides programmatic email inboxes designed for AI agents. The reply-all endpoint (`POST /v0/inboxes/{inbox_id}/messages/{message_id}/reply-all`) sends a reply to all recipients of an existing message within a specified inbox. The request body accepts optional fields including HTML/text body content, labels, custom headers, reply-to addresses, and attachments. On success it returns the new message_id and thread_id.
AgentMail's broader API covers inbox creation and management, message sending/receiving/forwarding/replying, threads, drafts, attachments, custom domains, webhooks, allow/block lists, and metrics. All operations are exposed over MPP (Micropayment Protocol) with fixed pricing. The reply-all operation is priced at $0.01 per call according to the OpenAPI x-payment-info. Authentication is via Bearer token in the Authorization header.
The probe did not capture a 402 MPP challenge on this specific endpoint (it returned 404 on HEAD/GET, which is expected since the operation is POST-only with path parameters). The OpenAPI spec is comprehensive with full request/response schemas. The root origin at mpp.api.agentmail.to serves the AgentMail landing page successfully (HTTP 200).
Capabilities
Use cases
- —AI agent replying to all participants in an email thread
- —Automated customer support responding to group conversations
- —Multi-party email coordination by autonomous agents
- —Workflow automation that needs to keep all original recipients in the loop
- —Agent-driven email campaigns with reply-all follow-ups
Fit
Best for
- —AI agents that need their own email inboxes with full reply-all capability
- —Developers building autonomous email workflows for agents
- —Applications requiring programmatic group email replies
Not for
- —Bulk marketing email blasts (not a mass-mailing service)
- —Human-facing email clients or webmail interfaces
- —Reading or searching emails without sending replies
Quick start
curl -X POST 'https://mpp.api.agentmail.to/v0/inboxes/{inbox_id}/messages/{message_id}/reply-all' \
-H 'Authorization: Bearer YOUR_TOKEN' \
-H 'Content-Type: application/json' \
-d '{"text": "Thanks everyone, acknowledged.", "html": "<p>Thanks everyone, acknowledged.</p>"}'Example
Request
{
"html": "<p>Thanks everyone, acknowledged.</p>",
"text": "Thanks everyone, acknowledged.",
"labels": [
"sent"
],
"attachments": []
}Response
{
"thread_id": "thr_xyz789",
"message_id": "msg_abc123"
}Endpoint
Quality
Full OpenAPI 3.1 spec with detailed schemas and pricing metadata is available. However, the specific endpoint returned 404 during probing (expected since it's POST-only with path params), so no live 402 MPP challenge was captured. Docs pages on the MPP gateway return 'Endpoint not found', so external documentation is sparse for this gateway specifically.
Warnings
- —No 402 MPP challenge was captured for this endpoint; it returned 404 on HEAD/GET because it only accepts POST with valid path parameters.
- —The price field in x-payment-info is '0.01' but the currency/token is not explicitly stated in the OpenAPI spec; assumed USD based on MPP conventions.
- —Documentation URLs on the MPP gateway (/docs, /api) return 'Endpoint not found'; refer to https://docs.agentmail.to/ for full docs.
Citations
- —The reply-all endpoint is POST /v0/inboxes/{inbox_id}/messages/{message_id}/reply-all with x-payment-info price of 0.01https://mpp.api.agentmail.to
- —AgentMail is the email inbox API for AI agents, backed by $6M seed fundinghttps://mpp.api.agentmail.to
- —The request body schema is ReplyAllMessageRequest with optional html, text, labels, headers, reply_to, and attachments fieldshttps://mpp.api.agentmail.to
- —The response schema is SendMessageResponse containing message_id and thread_idhttps://mpp.api.agentmail.to