Send an email from an AI agent's inbox via the AgentMail API, paid per-call over MPP.
What it does
This endpoint sends an email message from a specific AgentMail inbox. It is part of the AgentMail MPP gateway — a full email infrastructure API designed for AI agents. You POST a JSON body containing recipients (to, cc, bcc), subject, text/html body, optional attachments, headers, and reply-to addresses. On success it returns the new message_id and thread_id.
The endpoint lives at `POST /v0/inboxes/{inbox_id}/messages/send` and requires Bearer authentication. According to the OpenAPI spec's `x-payment-info`, the fixed price per call is 0.01 (the currency/unit is not explicitly stated in the spec but the broader AgentMail MPP gateway uses dollar-denominated pricing for other operations like $2.00 to create an inbox and $10.00 to create a domain). The endpoint supports to/cc/bcc as either a single string or an array of strings, HTML and plain-text bodies, custom headers, and file attachments (base64-encoded or by URL).
AgentMail is a YC-backed company that raised $6M in seed funding. The broader API also exposes inbox management, threads, drafts, webhooks, custom domains, allow/block lists, and metrics — all accessible through the same MPP gateway at mpp.api.agentmail.to.
Capabilities
Use cases
- —AI agent sends outbound emails on behalf of a user or organization
- —Automated customer support agent replies to inbound emails
- —Workflow automation that composes and sends email notifications
- —Agent-to-agent communication via dedicated email inboxes
- —Sending transactional emails with attachments from an AI pipeline
Fit
Best for
- —AI agents that need their own email addresses and outbound sending
- —Developers building agent workflows that include email communication
- —Pay-per-call email sending without monthly subscriptions
Not for
- —Bulk marketing email campaigns (no mention of bulk-send support)
- —Reading or receiving email (use the messages/list or webhooks endpoints instead)
- —Human-facing email clients or webmail UIs
Quick start
curl -X POST 'https://mpp.api.agentmail.to/v0/inboxes/{inbox_id}/messages/send' \
-H 'Authorization: Bearer <TOKEN>' \
-H 'Content-Type: application/json' \
-d '{
"to": "recipient@example.com",
"subject": "Hello from my agent",
"text": "This email was sent by an AI agent."
}'Example
Request
{
"cc": [
"bob@example.com"
],
"to": [
"alice@example.com"
],
"html": "<h1>Report</h1><p>Here is your weekly summary.</p>",
"text": "Report\nHere is your weekly summary.",
"subject": "Weekly Report",
"attachments": [
{
"content": "<base64-encoded-content>",
"filename": "report.pdf",
"content_type": "application/pdf",
"content_disposition": "attachment"
}
]
}Response
{
"thread_id": "thr_xyz789",
"message_id": "msg_abc123"
}Endpoint
Quality
Full OpenAPI 3.1 schema is available with detailed request/response types and x-payment-info pricing. However, the probe did not capture a live 402 MPP challenge on this specific endpoint (returned 404 on HEAD, 403 on GET — expected since it's a POST-only route). The price unit/currency is not explicitly labeled in the spec, requiring inference from context.
Warnings
- —Probe did not capture a 402 MPP challenge on this POST-only endpoint (HEAD returned 404, GET returned 403); the endpoint is likely live but liveness is not fully confirmed.
- —Price is listed as 0.01 in x-payment-info but the currency/token and decimals are not explicitly specified in the OpenAPI spec; inferred as ~$0.01 USD based on sibling endpoint pricing patterns.
Citations
- —The send message endpoint is POST /v0/inboxes/{inbox_id}/messages/send with fixed price 0.01 per callhttps://mpp.api.agentmail.to
- —Request body accepts to, cc, bcc, html, text, subject, headers, reply_to, and attachments fields per the SendMessageRequest schemahttps://mpp.api.agentmail.to
- —Response returns message_id and thread_id per the SendMessageResponse schemahttps://mpp.api.agentmail.to
- —Creating an inbox costs 2.000000 and creating a domain costs 10.000000 in the same pricing systemhttps://mpp.api.agentmail.to
- —AgentMail raised $6M in seed funding and is YC-backedhttps://mpp.api.agentmail.to