# AgentMail Reply To Message

> AgentMail Reply To Message is a paid API for AI agents from AgentMail at mpp.api.agentmail.to, paid per call via MPP, $0.01/call, status unknown (last checked 2026-10-12).

Sends a reply to a specific email message within an AgentMail inbox, supporting reply-all, CC/BCC, HTML/text bodies, attachments, and custom headers.

## Facts

- Endpoint: POST https://mpp.api.agentmail.to/v0/inboxes/:inbox_id/messages/:message_id/reply?utm_source=zero.xyz
- Price: $0.01/call
- Payment: MPP
- Status: unknown
- Last checked: 2026-10-12
- Activations on Zero: 0
- Provider: AgentMail
- Docs: https://docs.agentmail.to
- Website: https://agentmail.to
- Tags: email, inboxes, domains, drafts, threads, webhooks, messaging, ai, social
- Canonical page: https://www.zero.xyz/c/agentmail-reply-4671f3f6
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_0i6Rthr8GTEbFUP_L_CV4

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability agentmail-reply-4671f3f6 -d '<json body>'
```

Example prompt: Reply to the last email in inbox 'support-agent@agentmail.to' (inbox ID: inbox_abc123, message ID: msg_xyz789) with: 'Thanks for reaching out! We've received your request and will follow up within 24 hours.' — reply all and CC billing@mycompany.com.

## When to prefer this

Use this endpoint when an AI agent needs to respond to a specific received email message within an AgentMail-managed inbox. Prefer this over sending a new message when the context requires threading (reply-to headers, original message association). Ideal for automated customer support agents, AI email assistants, or any workflow that needs to close the loop on inbound messages.

## Known failure modes

- Invalid inbox_id or message_id returns 404 Not Found
- Missing or invalid Authorization Bearer token returns 401 Unauthorized
- Malformed request body (e.g. invalid email address format) returns 400 Bad Request
- Inbox not found or inaccessible returns 403 Forbidden
- Both html and text body fields absent may result in empty message error
- Invalid attachment URL or base64 content may cause attachment delivery failure

## How this service works

Reply To Message

## Output

Returns a JSON object containing the message_id of the newly sent reply and the thread_id of the email thread it belongs to, confirming the reply was delivered.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "title": "ReplyToMessageRequest",
 "properties": {
  "cc": {
   "oneOf": [
    {
     "type": "string"
    },
    {
     "type": "array",
     "items": {
      "type": "string"
     }
    }
   ],
   "title": "Addresses"
  },
  "to": {
   "oneOf": [
    {
     "type": "string"
    },
    {
     "type": "array",
     "items": {
      "type": "string"
     }
    }
   ],
   "title": "Addresses"
  },
  "bcc": {
   "oneOf": [
    {
     "type": "string"
    },
    {
     "type": "array",
     "items": {
      "type": "string"
     }
    }
   ],
   "title": "Addresses"
  },
  "html": {
   "type": "string",
   "title": "MessageHtml",
   "description": "HTML body of message."
  },
  "text": {
   "type": "string",
   "title": "MessageText",
   "description": "Plain text body of message."
  },
  "labels": {
   "type": "array",
   "items": {
    "type": "string"
   },
   "title": "MessageLabels",
   "description": "Labels of message."
  },
  "headers": {
   "type": "object",
   "title": "SendMessageHeaders",
   "description": "Headers to include in message.",
   "additionalProperties": {
    "type": "string"
   }
  },
  "reply_to": {
   "oneOf": [
    {
     "type": "string"
    },
    {
     "type": "array",
     "items": {
      "type": "string"
     }
    }
   ],
   "title": "Addresses"
  },
  "reply_all": {
   "type": "boolean",
   "title": "ReplyAll",
   "description": "Reply to all recipients of the original message."
  },
  "attachments": {
   "type": "array",
   "items": {
    "type": "object",
    "title": "SendAttachment",
    "properties": {
     "url": {
      "type": "string",
      "description": "URL to the attachment."
     },
     "content": {
      "type": "string",
      "description": "Base64 encoded content of attachment."
     },
     "filename": {
      "type": "string",
      "title": "AttachmentFilename",
      "description": "Filename of attachment."
     },
     "content_id": {
      "type": "string",
      "title": "AttachmentContentId",
      "description": "Content ID of attachment."
     },
     "content_type": {
      "type": "string",
      "title": "AttachmentContentType",
      "description": "Content type of attachment."
     },
     "content_disposition": {
      "enum": [
       "inline",
       "attachment"
      ],
      "type": "string",
      "title": "AttachmentContentDisposition",
      "description": "Content disposition of attachment."
     }
    }
   },
   "title": "SendMessageAttachments",
   "descr
… (truncated)
```

## Response schema (JSON Schema)

```json
{
 "type": "object",
 "title": "SendMessageResponse",
 "required": [
  "message_id",
  "thread_id"
 ],
 "properties": {
  "thread_id": {
   "type": "string",
   "title": "ThreadId",
   "description": "ID of thread."
  },
  "message_id": {
   "type": "string",
   "title": "MessageId",
   "description": "ID of message."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/agentmail-reply-4671f3f6/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from mpp.api.agentmail.to](https://www.zero.xyz/host/mpp.api.agentmail.to/llms.txt)