Comparison

zSender vs Postmark

An email API and SMTP relay hosted in the EU, compared to Postmark on data residency, pricing, SMTP and the API, and how to move over.

The short version

Postmark (an ActiveCampaign product) is a transactional email service with a REST API, webhooks, and SMTP, known for message streams that keep transactional and broadcast mail on separate sending infrastructure. It is a US-based company that stores its data, including the data it processes for customers, in the US. zSender is hosted in Germany, so the email data path stays in the EU by default. Postmark costs less to start at 10,000 emails a month; zSender has the larger free tier and EU residency.

zSender vs Postmark at a glance

Topic	zSender	Postmark
Hosting region	Hosted in Germany (EU). Your email data stays in the EU.	US-based; data stored in the US, no EU servers
Free tier	1,000 emails/month	100 emails/month (never expires)
Paid entry	$29/mo for 50,000 emails	Basic from $15/mo (10,000 emails)
SMTP relay	Yes: smtp.zsender.io (587/2525, STARTTLS)	Yes: SMTP on every plan (smtp.postmarkapp.com, 587)
API	REST API + webhooks	REST API + webhooks
GDPR / data residency	GDPR-oriented; recipient email processed in the EU	No EU data residency; EU transfers rely on Standard Contractual Clauses

Postmark details as of September 2026, from their own pricing and documentation pages. Check their site for current terms.

Data residency

EU data residency: the core difference

Postmark says plainly in its GDPR FAQ that it is a US-based company that stores its data in the US, including the data it processes on behalf of customers, answering a question that notes it has no servers in the EU. For EU customers it points to a transfer assessment under the Standard Contractual Clauses. Its SMTP service has endpoints in several AWS regions for low latency, but that is about where you connect; the FAQ still places the data in the US. zSender runs in Germany, so recipient addresses, message content, and delivery events are processed and stored in the EU.

The EU claim covers the email data path; billing and analytics use providers outside the EU, listed in our privacy policy.

Pricing

Pricing compared

Postmark is cheaper to start: Basic is $15/mo for 10,000 emails, with extra emails at $1.80 per 1,000, versus zSender's single $29/mo plan for 50,000 emails. Its free Developer plan is 100 emails/month and never expires; zSender's free tier is 1,000 emails/month. Which costs less depends on your volume: at 10,000 emails a month, Postmark does.

SMTP and API

SMTP relay and API

Both offer a REST API, webhooks, and SMTP. Postmark's SMTP server (smtp.postmarkapp.com, port 587) authenticates with a server API token; zSender's SMTP relay (smtp.zsender.io, ports 587/2525, STARTTLS) authenticates with your API key. Postmark's message streams, which run transactional and broadcast mail on parallel but separate infrastructure, have no zSender equivalent; if you send both kinds of mail and want them kept apart, that is a real Postmark advantage.

Why switch

Why teams look for a Postmark alternative

Postmark (an ActiveCampaign product) is an established transactional email service, and its message streams keep transactional and broadcast mail on parallel but separate sending infrastructure. What sends EU teams looking elsewhere is residency. Postmark's GDPR FAQ says it is a US-based company that stores its data in the US, including the data it processes on behalf of customers, answering a question that notes it has no servers in the EU, and for EU customers it points to a transfer assessment under the Standard Contractual Clauses. Its SMTP endpoints in several AWS regions change where you connect, not where the data is stored. If you would rather not rest your email data path on a transfer assessment, an EU-hosted provider removes that question. Postmark stays cheaper to start, at $15/mo for 10,000 emails.

Migration

Migrating from Postmark

Postmark authenticates API calls with a server API token in the X-Postmark-Server-Token header and sends with POST https://api.postmarkapp.com/email, where To is a comma-separated string (at most 50 recipients across To, Cc and Bcc combined). zSender takes the key as a Bearer token on POST https://api.zsender.io/v1/emails/send, with to, cc and bcc as arrays, each capped at 50. Postmark's MessageStream picks a stream and defaults to the outbound transactional one; zSender has no streams and sends all mail through the same infrastructure, so what a stream did for you splits in two. Mark 1:1 operational mail (a password reset, a receipt) with transactional: true, which skips the one-click List-Unsubscribe offer every other send gets. Carry Postmark's Tag over to zSender's tag, which drives filtering and a per-tag analytics breakdown. Over SMTP, Postmark's server token is both username and password, with an X-PM-Message-Stream header; on zSender the username is apikey, the password a zSender key, and X-Zsender-Transactional: true marks transactional mail.

If you send over SMTP, point your client at smtp.zsender.io with these settings. Send from a domain you have verified and your message code stays the same.

SMTP settings

host      smtp.zsender.io
port      587 or 2525 (STARTTLS)
username  apikey
password  your zSender API key

Which should you choose?

Choose Postmark if

You want the lowest entry price at around 10,000 emails a month, or separate streams for transactional and broadcast mail, and US data storage is acceptable.

Choose zSender if

The email data path must stay in the EU, and you want a larger free tier.

Try it on the free plan.

Get an API key

1,000 emails a month free, no card. Pro is $29 a month for 50,000.