Compatible API · SparkPost

SparkPost API compatible: change the URL and the key

SMTP Senpai accepts send requests in the same format as the SparkPost Transmissions API. Point your code to SMTP Senpai, use your msp_ key and keep your SDK, your fields and your error handling.

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Same paths, fields and errors
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Node.js · sparkpost 2.1.4 · what changes

  const SparkPost = require('sparkpost');
- const client = new SparkPost(process.env.SPARKPOST_API_KEY);
+ const client = new SparkPost(process.env.SMTP_SENPAI_KEY, { // msp_…
+   origin: 'https://app.mailsenpai.com/relay/compat/sparkpost',
+ });

what changes

Two things change, the rest stays

Same paths, same fields, same responses and error shapes as the SparkPost Transmissions API.

Base URLhttps://app.mailsenpai.com/relay/compat/sparkpost

Or the dedicated hosthttps://sparkpost-api.mailsenpai.com

Where the key goesAuthorization: msp_…

Paths (unchanged)POST /api/v1/transmissions

The sender must be on a domain verified in your SMTP Senpai area and signed with DKIM: otherwise the request is refused with an error in the SparkPost format. To try without sending, use options.sandbox: the request is checked and nothing leaves.

the code

Your code, pointed to SMTP Senpai

Node.js · sparkpost 2.1.4 · after

const SparkPost = require('sparkpost');
const client = new SparkPost(process.env.SMTP_SENPAI_KEY, { // msp_…
  origin: 'https://app.mailsenpai.com/relay/compat/sparkpost',
});

Tested with sparkpost 2.1.4: run with the official SDK against the SMTP Senpai endpoints.

Other languages and frameworks

Settings read in the source code of each SDK: they follow the same rule (base URL or host, plus your key). Try them with a test send before going live.

SDK	Setting	Status
PHP sparkpost/sparkpost	new SparkPost($http, ['key' => $key, 'host' => 'sparkpost-api.mailsenpai.com'])	Read in the SDK source code, not yet tested
Python sparkpost	SparkPost(key, base_uri='sparkpost-api.mailsenpai.com')	Read in the SDK source code, not yet tested

fields

What works, what is ignored, what is refused

Works

recipients (address, header_to, substitution_data), content.from, subject, text, html, reply_to, headers (CC), attachments, inline_images, email_rfc822, substitution_data ({{name}}), options.sandbox, Idempotency-Key

Accepted without effect

campaign_id, metadata, description, options.open_tracking, click_tracking, transactional, ip_pool

Named in the X-MailSenpai-Ignored response header.

Refused with the SparkPost error

content.template_id, recipients.list_id, options.start_time in the future (422)

Stored recipient lists and stored templates live on your side: send the recipients and the final content in the request.

events and webhooks

Delivery results from the SMTP Senpai API

The SparkPost endpoints for logs, events, statistics and webhooks are not reproduced. Results (sent, delivered, bounce, defer, complaint, dropped, open, click) come from the SMTP Senpai API with the same key: GET /relay/v1/eventi, or a signed webhook set in your SMTP Senpai area.

The id in the send response is the message_id of those events. Each response also carries X-MailSenpai-Result (sent, queued, suppressed, failed).

Read the events and webhook reference

Limits. Up to 50 recipients per message and 500 copies per request (over 50 they are queued and leave within about a minute), 10 attachments and 10 MB per message. Templates and scheduled sending are not supported: send the final content when you want it to leave.

Terminal · SMTP Senpai events

curl "https://app.mailsenpai.com/relay/v1/eventi?quanti=50" \
  -H "Authorization: Bearer $SMTP_SENPAI_KEY"

# {"ok": true, "eventi": [{"type": "delivered",
#   "email": "…", "message_id": "…", …}], "prossimo": …}

Frequently asked questions

Do I have to rewrite my SparkPost code?

No. You change the base URL (or the host) and the key. Paths, fields, responses and the error format stay those of the SparkPost Transmissions API, so your SDK and your error handling keep working.

What about templates and scheduled sending?

They are not supported: requests that use them are refused with the native error, so nothing leaves by mistake. Build the final content in your code and send it when you want it to leave.

How do I get bounces, opens and webhooks?

From the SMTP Senpai API, with the same key: GET /relay/v1/eventi or a signed webhook set in your SMTP Senpai area. The SparkPost event and webhook endpoints are not reproduced.

Can I test without sending?

Yes, as with SparkPost: set options.sandbox. The request is checked and nothing is sent.

Can I keep sending through SMTP?

Yes. The same account also works over SMTP (relay.mailsenpai.com, port 2525, STARTTLS) and with the SMTP Senpai REST API.

more compatible APIs

The other formats SMTP Senpai accepts

All 14 compatible APIs

The table with every base URL, dedicated host and key.

Open

SendGrid API compatible

SendGrid v3 Mail Send API

Open

Mailgun API compatible

Mailgun Messages API

Open

Amazon SES API compatible

Amazon SES API (v2 SendEmail, v1 SendEmail and SendRawEmail)

Open

Postmark API compatible

Postmark Email API

Open

Resend API compatible

Resend Emails API

Open

Try SMTP Senpai with your current code

14 days to change the base URL and the key, send your real emails and check every result in the log. Then decide.

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SparkPost is a trademark of the Bird group (formerly MessageBird). It is named only to describe the request format SMTP Senpai accepts. MailSenpai is not affiliated with SparkPost or Bird and this page is not approved by them. Information checked on 9 October 2026.

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