Reply to an email message in an AI agent's inbox via the AgentMail MPP API.
What it does
This endpoint lets an AI agent reply to a specific email message within an AgentMail inbox. It is part of the AgentMail MPP (Micropayment Protocol) gateway, which provides programmatic email inboxes purpose-built for AI agents. The reply operation is a POST to `/v0/inboxes/{inbox_id}/messages/{message_id}/reply` and accepts a JSON body with optional fields for text/HTML body, CC/BCC recipients, attachments, reply-to addresses, headers, and a `reply_all` flag. On success it returns the new `message_id` and `thread_id`.
The endpoint is priced at a fixed $0.01 per call according to the OpenAPI `x-payment-info` annotation (price "0.01", MPP protocol). The broader AgentMail API exposed through this MPP gateway covers inbox creation ($2.00), domain registration ($10.00), sending messages ($0.01), forwarding ($0.01), drafts, threads, webhooks, allow/block lists, and metrics. Authentication is via Bearer token. The API is documented with a full OpenAPI 3.1 spec served from the gateway origin.
Note: the probe did not capture a live 402 MPP challenge on this specific path (HEAD and GET returned 404, which is expected since the endpoint only accepts POST). The OpenAPI spec and the root landing page both confirm the service is operational. AgentMail is a YC-backed company that raised $6M in seed funding.
Capabilities
Use cases
- —AI agent replying to inbound customer emails automatically
- —Autonomous agent participating in multi-turn email conversations
- —Automated support bot responding to inquiries in its own inbox
- —Agent-to-agent communication via email threads
- —Workflow automation that replies to form submissions or notifications
Fit
Best for
- —AI agents that need their own email addresses and can reply to messages
- —Developers building autonomous email workflows for bots
- —Multi-turn conversational email threads managed by agents
Not for
- —Bulk marketing email campaigns (this is per-message, agent-oriented)
- —Reading or managing a human user's existing Gmail/Outlook inbox
Quick start
curl -X POST https://mpp.api.agentmail.to/v0/inboxes/{inbox_id}/messages/{message_id}/reply \
-H "Authorization: Bearer $TOKEN" \
-H "Content-Type: application/json" \
-d '{"text": "Thanks for your email. I will follow up shortly."}'Example
Request
{
"html": "<p>Thanks for your email. I will follow up shortly.</p>",
"text": "Thanks for your email. I will follow up shortly.",
"reply_all": false,
"attachments": []
}Response
{
"thread_id": "thr_xyz789",
"message_id": "msg_abc123"
}Endpoint
Quality
Full OpenAPI 3.1 schema is available with detailed request/response types and pricing annotations. However, the probe did not capture a live 402 MPP challenge on this endpoint (POST-only route probed with HEAD/GET), and no separate documentation pages were reachable from the MPP gateway. Pricing is inferred from x-payment-info in the spec.
Warnings
- —Probe returned 404 because HEAD/GET were used on a POST-only endpoint; the endpoint is likely live but a POST-based 402 challenge was not captured.
- —No dedicated documentation pages were found on the MPP gateway host (docs, api, pricing all returned 'Endpoint not found').
- —Price currency/denomination for MPP payments is not explicitly stated in the spec; $0.01 is inferred from the 'price: 0.01' annotation assuming USD.
Citations
- —The reply endpoint is POST /v0/inboxes/{inbox_id}/messages/{message_id}/reply with x-payment-info price 0.01https://mpp.api.agentmail.to
- —AgentMail raised $6M in Seed Fundinghttps://mpp.api.agentmail.to
- —AgentMail is described as 'Email Inboxes for AI Agents'https://mpp.api.agentmail.to
- —Create Inbox is priced at 2.000000 and Create Domain at 10.000000 per the OpenAPI spechttps://mpp.api.agentmail.to