Sendmux Send Email

Sendmux send email

Use this skill when the user is ready to send outbound email through Sendmux or needs code/commands for sending.

Safety first

• Do not ask the user to paste an API key.
• Do not invent recipients, sender addresses, subject lines, or body content.
• Send only after the user supplies or confirms every recipient and message.
• For batch sends, confirm the full recipient/message set before calling a send tool.
• Use a send-capable smx_mbx_ key or owner-approved Sending-resource smx_agent_ token for the Sending API.
• Do not use a pre-claim smx_agent_ token for sending. Pre-claim self-registered agent tokens have mailbox.read and email.receive, not email.send.
• If using a self-registered agent, obtain the Sending-resource smx_agent_ token by exchanging the saved claim_token after owner approval with resource=https://smtp.sendmux.ai/api/v1.

Choose the send path

Batch sends accept up to 100 messages. A batch response can partially succeed, so inspect every result item.

Required JSON shape

Single send body:

{
  "from": { "email": "sender@example.com", "name": "Sender Name" },
  "to": { "email": "recipient@example.com", "name": "Recipient Name" },
  "subject": "Subject line",
  "html_body": "<p>Hello.</p>",
  "text_body": "Hello."
}

Required fields: from, to, subject, html_body.

Useful optional fields:

• text_body: plain text alternative.
• cc, bcc: arrays of recipients, max 100 each.
• reply_to: one address object.
• return_path: envelope sender for bounce handling.
• custom_headers: custom X-* headers.
• attachments: up to 10 items. Prefer uploaded refs { "attachment_id": "att_..." }. Inline compatibility form uses filename plus base64 content, optional type, and encoding: "base64".

For real local files, do not ask the model to produce base64. Route attachment-heavy work to sendmux-attachments; use CLI --attach, SDK file helpers, sending_upload_attachment, or delegated sending_create_attachment_upload, then pass attachment_id refs. Sending uploads cap each file at 18 MiB and the final sent message at 25 MB. Mailbox blob_id refs are for mailbox sends, not Sending API sends.

Batch send body:

{
  "messages": [
    {
      "from": { "email": "sender@example.com" },
      "to": { "email": "alice@example.com" },
      "subject": "Hello Alice",
      "html_body": "<p>Hi Alice.</p>"
    },
    {
      "from": { "email": "sender@example.com" },
      "to": { "email": "bob@example.com" },
      "subject": "Hello Bob",
      "html_body": "<p>Hi Bob.</p>"
    }
  ]
}

Idempotency

Add Idempotency-Key to every send that may be retried. Use one stable key per logical email or batch.

• Same key and same body: returns the cached response for 24 hours.
• Same key and different body: returns 409 idempotency_conflict.
• Keep keys under 255 characters.

MCP

Use MCP when the user's agent already has the Sending server connected:

• One message: sending_send_email.
• Multiple messages: sending_send_email_batch.
• Local file upload before send: sending_upload_attachment with file_path.
• Hosted or shell-capable upload before send: sending_create_attachment_upload, then PUT bytes to the returned URL with the returned headers and no Sendmux API key.
• Metadata check for a temporary uploaded attachment: sending_get_attachment.

Include an idempotency key when the client exposes the header parameter. If the MCP client does not expose headers clearly, use CLI, SDK, or direct HTTP for retry-sensitive sends. For attachments through MCP, use sendmux-attachments so the agent chooses file_path, presigned upload, or tiny inline base64 correctly.

CLI

One email:

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux sending:send \
  --idempotency-key "$IDEMPOTENCY_KEY" \
  --body '{
    "from": { "email": "sender@example.com", "name": "Sender Name" },
    "to": { "email": "recipient@example.com", "name": "Recipient Name" },
    "subject": "Subject line",
    "html_body": "<p>Hello.</p>",
    "text_body": "Hello."
  }' \
  --json

Batch:

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux sending:send:batch \
  --idempotency-key "$IDEMPOTENCY_KEY" \
  --body-file ./sendmux-batch.json \
  --json

Use --attach ./file.pdf for local files; the CLI uploads bytes first and injects attachment_id refs before sending. Use --body-file for larger JSON payloads or already-prepared Sending API attachment objects.

TypeScript SDK

One email:

import { createSendingClient, sendingSendEmail } from "@sendmux/sending";

const client = createSendingClient({ apiKey: process.env.SENDMUX_API_KEY! });

const response = await sendingSendEmail({
  client,
  headers: { "Idempotency-Key": idempotencyKey },
  body: {
    from: { email: "sender@example.com", name: "Sender Name" },
    to: { email: "recipient@example.com", name: "Recipient Name" },
    subject: "Subject line",
    html_body: "<p>Hello.</p>",
    text_body: "Hello.",
  },
});

console.log(response.data.message_id, response.data.status);

Batch:

import { createSendingClient, sendingSendEmailBatch } from "@sendmux/sending";

const client = createSendingClient({ apiKey: process.env.SENDMUX_API_KEY! });

const response = await sendingSendEmailBatch({
  client,
  headers: { "Idempotency-Key": idempotencyKey },
  body: {
    messages,
  },
});

for (const result of response.data.results) {
  console.log(result.index, result.status, result.message_id, result.error);
}

Direct HTTP

Use direct HTTP only when MCP, CLI, or SDK is unavailable:

curl -X POST https://smtp.sendmux.ai/api/v1/emails/send \
  -H "Authorization: Bearer $SENDMUX_MBX_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $IDEMPOTENCY_KEY" \
  -d @sendmux-email.json

Responses and errors

Single send success returns a 200 success envelope with:

• data.message_id: eml_...
• data.status: queued

Batch success returns a 200 success envelope with:

• data.summary.total
• data.summary.queued
• data.summary.failed
• data.results[] containing index, status, message_id, and optional error

Handle these errors deliberately:

• 401: missing, invalid, or revoked key.
• 402: insufficient credits.
• 403: key lacks email.send, is not allowed for the sender, or uses the wrong surface.
• 409: idempotency conflict.
• 413: request body exceeds 25 MB.
• 422: validation failed; read error.errors.
• 429 or 503: retry according to response headers.

Routing

• Setup/auth first call: sendmux-getting-started.
• Attachment file paths, presigned upload URLs, and download URLs: sendmux-attachments.
• Mailbox-centred replies: sendmux-mailbox-agent.
• CLI-only details: sendmux-cli.
• Cheapest-call decisions: sendmux-token-efficient-usage.