Compatible API · Mandrill

Mandrill API compatible: change the URL and the key

SMTP Senpai accepts send requests in the same format as the Mandrill / Mailchimp Transactional 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
Sending servers and log in the EU
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Node.js · @mailchimp/mailchimp_transactional 1.4.1 · what changes

- const mailchimp = require('@mailchimp/mailchimp_transactional')(process.env.MANDRILL_API_KEY);
+ const mailchimp = require('@mailchimp/mailchimp_transactional')(process.env.SMTP_SENPAI_KEY); // msp_…
+ mailchimp.basePath = 'https://app.mailsenpai.com/relay/compat/mandrill/api/1.3';

what changes

Two things change, the rest stays

Same paths, same fields, same responses and error shapes as the Mandrill / Mailchimp Transactional API.

Base URLhttps://app.mailsenpai.com/relay/compat/mandrill/api/1.0

Or the dedicated hosthttps://mandrill-api.mailsenpai.com/api/1.0

Where the key goes"key": "msp_…" in the JSON body

Paths (unchanged)POST /messages/send.json, /messages/send-raw.json, /users/ping

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 Mandrill format.

the code

Your code, pointed to SMTP Senpai

Node.js · @mailchimp/mailchimp_transactional 1.4.1 · after

const mailchimp = require('@mailchimp/mailchimp_transactional')(process.env.SMTP_SENPAI_KEY); // msp_…
mailchimp.basePath = 'https://app.mailsenpai.com/relay/compat/mandrill/api/1.3';

Tested with @mailchimp/mailchimp_transactional 1.4.1: 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 mailchimp/transactional	$client->setHost('https://app.mailsenpai.com/relay/compat/mandrill/api/1.3');	Read in the SDK source code, not yet tested
Python mailchimp_transactional	client.api_client.host = 'https://app.mailsenpai.com/relay/compat/mandrill/api/1.3'	Read in the SDK source code, not yet tested

fields

What works, what is ignored, what is refused

Works

key, message.from_email, from_name, to (to/cc/bcc), subject, html, text, headers (Reply-To), attachments, images, preserve_recipients, bcc_address, global_merge_vars / merge_vars (*|NAME|* or handlebars); messages/send-raw; users/ping, users/ping2; versions 1.0, 1.3, 1.4, with or without .json

Accepted without effect

tags, metadata, track_*, subaccount, ip_pool, async, important

Named in the X-MailSenpai-Ignored response header.

Refused with the Mandrill error

messages/send-template, send_at

Versions 1.0, 1.3 and 1.4 of the API path work, with or without the .json suffix, so the official SDKs and plain HTTP clients both fit.

events and webhooks

Delivery results from the SMTP Senpai API

The Mandrill 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 Mandrill code?

No. You change the base URL (or the host) and the key. Paths, fields, responses and the error format stay those of the Mandrill / Mailchimp Transactional 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 Mandrill event and webhook endpoints are not reproduced.

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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Mailchimp and Mandrill are trademarks of The Rocket Science Group LLC, an Intuit company. It is named only to describe the request format SMTP Senpai accepts. MailSenpai is not affiliated with Mailchimp or Intuit and this page is not approved by them. Information checked on 9 October 2026.

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