Postmark setup

Transactional-grade delivery with a real suppression API. Postmark splits mail into message streams, and Mimeo routes every send to the right one automatically.

What you need

A Server API token (Postmark → your server → API Tokens tab).
A confirmed sender signature, or a verified DKIM domain. Postmark refuses every send without one. New Postmark accounts can also only deliver to addresses on the sender's own domain until Postmark approves the account.
A Broadcasts-type message stream (your server → Message Streams). Servers start with only the transactional outbound stream and an inbound stream — create a Broadcasts stream if there isn't one, and note its ID.
A webhook secret — any long random value you generate. Postmark can't sign webhooks, so this secret in the webhook URL is how your instance knows a request is Postmark's.
Optionally, an Account API token (Postmark → Account → API Tokens). With it, Verify connection also confirms a sender signature is actually confirmed, instead of leaving that to your first send.

Setting it up

Confirm your sender signature in Postmark, and create the Broadcasts-type stream.
In Mimeo: Settings → Provider → Postmark → Credentials. Paste the server token, the broadcast stream ID, your webhook secret, and (optionally) the account token.
Press Verify connection. It validates the token, checks the configured stream exists and really is a Broadcasts-type stream, and flags anything on the Postmark side that's configured against the grain.
Under Duties, set Postmark as the one-off sender.
Set up webhooks (below).

Credentials are encrypted in your instance's database. Never in a config file, never in an environment variable — so switching providers is a Settings change rather than a deploy.

Message streams

Postmark requires marketing mail to ride a Broadcasts-type stream, and keeps transactional mail on the default outbound stream. Mimeo routes automatically: broadcasts, sequences, flows and test sends go to your configured broadcast stream; transactional mail — password resets, operator alerts — rides outbound. Nothing to choose per send.

Webhooks

Postmark states plainly that it can't sign webhooks, so verification is the webhook secret carried in the URL. Webhooks are configured per stream:

Postmark → your server → your broadcast stream → Settings → Webhooks → Add webhook.
Point it at https://your-instance.com/webhooks/email/postmark?secret=YOUR_WEBHOOK_SECRET — the same value you entered in Credentials.
Check Delivery, Bounce, Spam Complaint and Subscription Change. Leave Open and Click unchecked — tracking is Mimeo's.
Add the same webhook on the outbound stream (Bounce and Delivery) if you want delivery truth for transactional mail too.

A request with a wrong or missing secret is refused. Rotate the secret by updating both sides.

Leave open and click tracking off

Postmark can track opens and clicks at the server level. Don't turn it on. The adapter forces tracking off on every message it sends anyway, and Verify connection flags a server that has it switched on. Tracking is Mimeo's — one dataset, with bot filtering, tied to the exact send.

The unsubscribe links

Broadcast streams default to Postmark-managed unsubscribe handling: Postmark appends its own unsubscribe link to the footer of every broadcast-stream message and replaces the RFC 8058 unsubscribe headers with its own. Mimeo's emails already carry their own unsubscribe link, so recipients see two.

Nothing breaks. An unsubscribe through Postmark's page comes straight back via the Subscription Change webhook and suppresses the person locally, so the two links agree. But the clean setup is one link — yours:

Ask Postmark support to enable custom unsubscribe handling for your account. It's permission-gated — the API refuses to set it self-serve.
Set the broadcast stream to Manage unsubscribes on your own.

Postmark then injects nothing: one visible unsubscribe link (Mimeo's), and Mimeo's one-click headers ride through intact, attributing every unsubscribe to the exact email it came from. Until that's done, Verify connection reminds you.

What Postmark does, and what Mimeo does

Postmark	Mimeo
Delivers the message	Decides who gets it and when
Reports delivery, bounces, complaints and subscription changes by webhook	Owns the suppression list and checks it before every send
Keeps a per-stream suppression list Mimeo reads and writes	Compiles the HTML, from your own layouts
—	Tracks opens and clicks
—	Sets the RFC 8058 unsubscribe headers, and hosts the one-click target
—	Paces the sending

Suppression flows both ways

An unsubscribe in Mimeo is pushed to Postmark as a manual suppression on the broadcast stream, so Postmark's own list agrees with yours. And Postmark's suppression list can be read back per address, which keeps the periodic suppression checks running — a suppression that happens Postmark-side doesn't stay a secret. One asymmetry worth knowing: Postmark refuses to delete spam-complaint suppressions, so a resubscribe here can't clear one there. The standing guard and Postmark's list both keep that address unmailed, which is the right outcome for a complaint.

No bulk pipeline — which is fine

Postmark has no broadcast-campaign API, so Mimeo can't hand a broadcast off to it. Broadcasts fan out through the ordinary paced queue instead. That's the better path anyway: guards apply, send windows apply, tracking applies, and every broadcast gets a full report.

Switching to Postmark from another provider

It's a Settings change. Enter the credentials, verify, reassign the duties. Your people, suppression list, flows, sequences, emails and history all stay exactly where they are — none of it was ever the provider's.