Compatible API · Postal

Postal API compatible: change the URL and the key

SMTP Senpai accepts send requests in the same format as the Postal send 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 · @atech/postal 1.0.0 · what changes

  const Postal = require('@atech/postal');
- const client = new Postal.Client('postal.yourcompany.com', process.env.POSTAL_API_KEY);
+ const client = new Postal.Client('postal-api.mailsenpai.com', process.env.SMTP_SENPAI_KEY); // msp_…

what changes

Two things change, the rest stays

Same paths, same fields, same responses and error shapes as the Postal send API.

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

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

Where the key goesX-Server-API-Key: msp_…

Paths (unchanged)POST /api/v1/send/message, /api/v1/send/raw

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 Postal format.

the code

Your code, pointed to SMTP Senpai

Node.js · @atech/postal 1.0.0 · after

const Postal = require('@atech/postal');
const client = new Postal.Client('postal-api.mailsenpai.com', process.env.SMTP_SENPAI_KEY); // msp_…

Tested with @atech/postal 1.0.0: tested: connection 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 postal/postal	new \Postal\Client('https://postal-api.mailsenpai.com', $key)	Read in the SDK source code, not yet tested
Ruby postal-ruby	Postal::Client.new('https://postal-api.mailsenpai.com', key)	Read in the SDK source code, not yet tested

fields

What works, what is ignored, what is refused

Works

JSON or params= form bodies, to, cc, bcc, from, subject, reply_to, plain_body, html_body, headers, attachments; raw with mail_from, rcpt_to, data

Accepted without effect

tag, bounce, sender

Named in the X-MailSenpai-Ignored response header.

Refused with the Postal error

—

The Postal clients take a host name only: use the dedicated host. As in Postal, the HTTP status is always 200 and the result is in status.

events and webhooks

Delivery results from the SMTP Senpai API

The Postal 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 Postal code?

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

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Resend API compatible

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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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Postal is open-source software (MIT licence, © Krystal Hosting Ltd and contributors). It is named only to describe the request format SMTP Senpai accepts. MailSenpai is not affiliated with the Postal project or Krystal and this page is not approved by them. Information checked on 9 October 2026.

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