Compatible API · Resend

Resend API compatible: change the URL and the key

SMTP Senpai accepts send requests in the same format as the Resend Emails 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 · resend 6.32.1 · what changes

  import { Resend } from 'resend';
- const resend = new Resend(process.env.RESEND_API_KEY);
+ const resend = new Resend(process.env.SMTP_SENPAI_KEY, { // msp_…
+   baseUrl: 'https://app.mailsenpai.com/relay/compat/resend',
+ });

what changes

Two things change, the rest stays

Same paths, same fields, same responses and error shapes as the Resend Emails API.

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

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

Where the key goesAuthorization: Bearer msp_…

Paths (unchanged)POST /emails, /emails/batch

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 Resend format.

the code

Your code, pointed to SMTP Senpai

Node.js · resend 6.32.1 · after

import { Resend } from 'resend';
const resend = new Resend(process.env.SMTP_SENPAI_KEY, { // msp_…
  baseUrl: 'https://app.mailsenpai.com/relay/compat/resend',
});

Tested with resend 6.32.1: run with the official SDK against the SMTP Senpai endpoints.

Python · resend 2.49.1 · after

import os
import resend
resend.api_key = os.environ['SMTP_SENPAI_KEY']  # msp_…
resend.api_url = 'https://app.mailsenpai.com/relay/compat/resend'

Tested with resend 2.49.1 against SMTP Senpai: connection, authentication and error handling.

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 resend/resend-php	RESEND_BASE_URL=https://app.mailsenpai.com/relay/compat/resend	Read in the SDK source code, not yet tested
Go resend-go	client.BaseURL, _ = url.Parse("https://app.mailsenpai.com/relay/compat/resend/")	Read in the SDK source code, not yet tested

fields

What works, what is ignored, what is refused

Works

from, to, cc, bcc, reply_to, subject, html, text, headers, attachments (base64 content or Buffer, content_id), Idempotency-Key, x-batch-validation

Accepted without effect

tags, topic_id

Named in the X-MailSenpai-Ignored response header.

Refused with the Resend error

template, scheduled_at, attachments by remote path (422)

With the Node SDK you can also set the environment variable RESEND_BASE_URL instead of the baseUrl option. In Go the base URL needs a trailing slash.

events and webhooks

Delivery results from the SMTP Senpai API

The Resend 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 (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 Resend code?

No. You change the base URL (or the host) and the key. Paths, fields, responses and the error format stay those of the Resend Emails 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 Resend 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

Brevo API compatible

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

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