Compatible API · Postmark

Postmark API compatible: change the URL and the key

SMTP Senpai accepts send requests in the same format as the Postmark 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
Sending servers and log in the EU
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Node.js · postmark 5.1.0 · what changes

  const postmark = require('postmark');
- const client = new postmark.ServerClient(process.env.POSTMARK_SERVER_TOKEN);
+ const client = new postmark.ServerClient(process.env.SMTP_SENPAI_KEY, { // msp_…
+   requestHost: 'app.mailsenpai.com/relay/compat/postmark',
+ });

what changes

Two things change, the rest stays

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

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

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

Where the key goesX-Postmark-Server-Token: msp_…

Paths (unchanged)POST /email, /email/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 Postmark format.

the code

Your code, pointed to SMTP Senpai

Node.js · postmark 5.1.0 · after

const postmark = require('postmark');
const client = new postmark.ServerClient(process.env.SMTP_SENPAI_KEY, { // msp_…
  requestHost: 'app.mailsenpai.com/relay/compat/postmark',
});

Tested with postmark 5.1.0: 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 wildbit/postmark-php	\Postmark\PostmarkClientBase::$BASE_URL = 'https://app.mailsenpai.com/relay/compat/postmark';	Read in the SDK source code, not yet tested
.NET Postmark	new PostmarkClient(key, "https://app.mailsenpai.com/relay/compat/postmark/")	Read in the SDK source code, not yet tested
Ruby postmark	Postmark::ApiClient.new(key, host: 'app.mailsenpai.com', path_prefix: '/relay/compat/postmark/')	Read in the SDK source code, not yet tested
Symfony Mailer	MAILER_DSN=postmark+api://[email protected]	Read in the SDK source code, not yet tested

fields

What works, what is ignored, what is refused

Works

From, To, Cc, Bcc, Subject, HtmlBody, TextBody, ReplyTo, Headers, Attachments (ContentID), MessageStream: outbound; batch with one result per message, in order

Accepted without effect

Tag, Metadata, TrackOpens, TrackLinks, InlineCss

Named in the X-MailSenpai-Ignored response header.

Refused with the Postmark error

TemplateId, /email/withTemplate, /email/batchWithTemplates (422, ErrorCode 1101)

Error codes follow Postmark: 10 key, 300 invalid request, 400 sender domain, 406 all recipients suppressed, 410 batch over 500, 411 attachment type. The PHP SDK base URL is static: it applies to the whole process.

events and webhooks

Delivery results from the SMTP Senpai API

The Postmark 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 Postmark code?

No. You change the base URL (or the host) and the key. Paths, fields, responses and the error format stay those of the Postmark 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 Postmark 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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Mailgun API compatible

Mailgun Messages API

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Amazon SES API compatible

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

Open

Resend API compatible

Resend Emails 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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Postmark is a trademark of the ActiveCampaign group (AC PM, LLC). It is named only to describe the request format SMTP Senpai accepts. MailSenpai is not affiliated with Postmark or ActiveCampaign and this page is not approved by them. Information checked on 9 October 2026.

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