Postmark ships with a feature most operators discover only after they’ve already mixed password resets with product announcements: message streams. They let you route different types of email through separate pipelines, each with its own delivery tracking, suppression list, and sender reputation.
If you’re running both transactional email—receipts, login links, account notifications—and broadcast messages like product updates or weekly digests, message streams keep the two from contaminating each other’s deliverability.
What message streams actually do
Every Postmark account starts with two default streams: Transactional and Broadcasts. When you send an email via API or SMTP, you specify which stream it belongs to. Postmark then tracks opens, clicks, bounces, and spam complaints separately for each stream.
This separation matters because transactional email—password resets, order confirmations—typically sees open rates above 60% and near-zero spam complaints. Marketing broadcasts might hit 20% opens and attract a handful of complaints, even when people opted in. If you mix them in one stream, a spike in broadcast complaints can drag down your overall sender reputation, which affects all your email, including the critical transactional stuff.
Each stream also maintains its own suppression list. If someone marks your newsletter as spam, Postmark adds their address to the Broadcasts suppression list—but they’ll still receive password resets and receipts from the Transactional stream. You don’t lose the ability to send account-critical email just because someone unsubscribed from marketing.
When to create custom streams
Beyond the two defaults, you can create additional streams for specific use cases. Here are three that make sense for solo operators and small teams:
Onboarding sequences. If you run a multi-email onboarding series—welcome, getting-started tips, feature walkthroughs—route it through a dedicated stream. Onboarding email sits between transactional and broadcast: it’s expected, but not urgent. Separating it lets you monitor completion rates and deliverability without muddying your core transactional metrics.
Digest emails. Weekly or monthly roundups often see lower engagement than one-off broadcasts. A separate stream lets you track digest-specific open rates and adjust frequency without affecting your main broadcast reputation.
Partner or affiliate sends. If you occasionally send email on behalf of a partner—joint webinars, co-marketing—isolate it. Partner sends introduce variables you don’t control: list quality, subject lines, content. A separate stream quarantines the risk.
Postmark allows up to ten streams per account. You don’t pay extra for them, but each stream requires its own API token and SMTP credentials, so there’s a small setup cost.
How to route messages to the right stream
If you’re using Postmark’s API, you specify the stream with a MessageStream parameter in your JSON payload. For SMTP, you set the stream by choosing the correct SMTP credentials during configuration—each stream generates its own username and password.
Most developers default to the Transactional stream for everything, then wonder why their welcome emails show up in spam. The fix: audit every email type your app sends, classify it as transactional or broadcast, and route accordingly. Receipts, password resets, and two-factor codes go to Transactional. Product updates, newsletters, and nurture sequences go to Broadcasts or a custom stream.
The non-obvious tip: use streams to test reputation recovery
If your broadcast deliverability tanks—inbox placement drops, spam complaints spike—create a new message stream, warm it with a small segment of your most engaged subscribers, and migrate your broadcast sends over two weeks. The new stream starts with a clean reputation. You can’t erase your domain’s history, but you can isolate future sends from past damage.
This works because Postmark treats each stream as a separate sender profile. ISPs still see your domain and IP, but the engagement patterns and complaint rates reset. It’s not a magic fix—if your content or list quality is broken, the new stream will degrade just as fast—but it buys you time to tighten your targeting and content.
One warning: don’t create streams just to dodge suppression lists. If someone complained about your email, they don’t want any of your marketing, regardless of which stream it comes from. Routing around suppressions will get your account suspended.
If you’re already using Postmark and haven’t set up separate streams for transactional and broadcast email, do it this week. The deliverability buffer alone justifies the ten minutes of setup. And if you’re evaluating Postmark against other ESPs, message streams are one of the features that separate it from basic SMTP relays.
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