Compatible API · MailerSend

MailerSend API compatible: change the URL and the key

SMTP Senpai accepts send requests in the same format as the MailerSend Email 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 · mailersend 3.4.1 · what changes

  import { MailerSend } from 'mailersend';
- const mailerSend = new MailerSend({ apiKey: process.env.MAILERSEND_API_KEY });
+ const mailerSend = new MailerSend({ apiKey: process.env.SMTP_SENPAI_KEY }); // msp_…
+ // no official option: internal property, check it when you update the SDK
+ mailerSend.email.baseUrl = 'https://app.mailsenpai.com/relay/compat/mailersend/v1';

what changes

Two things change, the rest stays

Same paths, same fields, same responses and error shapes as the MailerSend Email API.

Base URLhttps://app.mailsenpai.com/relay/compat/mailersend/v1

Or the dedicated hosthttps://mailersend-api.mailsenpai.com/v1

Where the key goesAuthorization: Bearer msp_…

Paths (unchanged)POST /email, /bulk-email

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 MailerSend format.

the code

Your code, pointed to SMTP Senpai

Node.js · mailersend 3.4.1 · after

import { MailerSend } from 'mailersend';
const mailerSend = new MailerSend({ apiKey: process.env.SMTP_SENPAI_KEY }); // msp_…
// no official option: internal property, check it when you update the SDK
mailerSend.email.baseUrl = 'https://app.mailsenpai.com/relay/compat/mailersend/v1';

Tested with mailersend 3.4.1: 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 mailersend/mailersend	new MailerSend(['api_key' => $key, 'host' => 'app.mailsenpai.com/relay/compat/mailersend'])	Read in the SDK source code, not yet tested
Python mailersend	MailerSendClient(api_key=key, base_url='https://app.mailsenpai.com/relay/compat/mailersend/v1/')	Read in the SDK source code, not yet tested

fields

What works, what is ignored, what is refused

Works

from, to (1–50), cc, bcc (up to 10), reply_to, subject, text, html, attachments (inline with id), headers, personalization ({{ var }}), precedence_bulk, in_reply_to, references, list_unsubscribe

Accepted without effect

tags, settings

Named in the X-MailSenpai-Ignored response header.

Refused with the MailerSend error

template_id, send_at (422)

The Node SDK has no official option for the base URL: the line above sets an internal property that may change between SDK versions. The Go SDK has no option at all (only a custom http.Client). GET /bulk-email/{id} is not available: read the SMTP Senpai events instead.

events and webhooks

Delivery results from the SMTP Senpai API

The MailerSend 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 (X-Message-Id) 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 MailerSend code?

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

Is the Node SDK setting official?

No: the Node SDK has no option for the base URL, so the snippet sets an internal property. Check it when you update the SDK, or call the HTTP API directly.

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

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MailerSend is a trademark of MailerSend, Inc.. It is named only to describe the request format SMTP Senpai accepts. MailSenpai is not affiliated with MailerSend and this page is not approved by them. Information checked on 9 October 2026.

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