Switching from Postmark, ErrorCodes intact

Postmark integrations lean on semantics — ErrorCode 0, the 406 inactive recipient, batch always answering 200 per-message. Those survive the switch. This page is precise about what does not.

Facts verified 2026-08-29 — corrections: hello@emailfast.dev

Keep the semantics you built on

  import { ServerClient } from "postmark";

- const client = new ServerClient(process.env.POSTMARK_SERVER_TOKEN);
+ const client = new ServerClient(process.env.EMAILFAST_API_KEY, {
+   requestHost: "api.emailfast.dev", // ef_sandbox_… works everywhere
+ });

That's the code change, because SendGrid-, Mailgun-, and Postmark-compatible endpoints: point your existing SDK at a new base URL with a new key and keep your code — attachment sends are the one exception, refused with a clear error in your provider's own format rather than silently dropped. What the Postmark-compatible endpoints honor, verified against the code that serves them:

• POST /email and POST /email/batch, same shape, with your Email Fast key in the X-Postmark-Server-Token header.
• ErrorCode semantics preserved: 0 on success with a MessageID, 406 for an inactive (suppressed) recipient, 300 for a blocked send. A failing single send answers HTTP 422 with the ErrorCode in the body — real wire parity, so the official postmark npm client's typed errors work against it.
• Cc and Bcc carry over to every message fanned from a comma-separated To, with native guardrails: per-recipient suppression and quota, the same 50-recipient cap Postmark enforces, and BCC never appearing in a header.
• Batch behaves like Postmark's batch: always HTTP 200, one result per message, so a partial batch reads exactly the way your code already handles it — each element independently succeeds or fails.
• Attachments are refused loudly. Outbound attachments aren't supported yet, platform-wide; a send carrying one answers HTTP 422 with ErrorCode 300 and a message saying so — never a message quietly delivered without its file. Headers, Metadata, and InlineCss are accepted without effect. (Postmark's API has no scheduling field, so nothing arrives and nothing is dropped.)
• Retries can be made idempotent with an Idempotency-Key header, per recipient: a 202 from the API means the send is committed to a durable, partitioned outbox before we answer — a crash can't lose it, and a retry with the same idempotency key can't double-send.

And the same admission discipline Postmark users switch providers to keep applies to every ingress here: every send — REST, SMTP, browser SDK, compatibility endpoints, broadcasts, automations — passes through one admission gate: idempotency, suppression, quota, and content policy in a single checkpoint no ingress can skip.

What moves by hand — said plainly

There is no hosted Postmark importer. The planned import lane covers SendGrid, Mailgun, and Mailchimp; Postmark templates and layouts are re-created in the dashboard or over POST /v1/templates — slug each with its old TemplateId or alias so existing template sends resolve unchanged. Suppression lists export from Postmark as CSV and import here as suppressions — do that before your first send so old bounces and unsubscribes stay honored.

All of it runs dry first: sandbox keys (ef_sandbox_…) that run the real pipeline dry: real validation, real rendering, real events, a hosted capture inbox — and no email leaves.

What Postmark publishes at your volume

Postmark deserves credit for publishing exact calculator ticks rather than "starting at" prices — the table below is their own published points, not our interpolation.

Detail: Free developer tier: 100 emails/month, never expires. Calculator slider snaps to fixed ticks only: 10k/50k/125k/300k/700k/1.5M (no 100k or 500k stop) - 100k and 500k figures below use the smallest published tier that covers the volume. Three feature levels at every volume: Basic, Pro (+10%, e.g. $16.50 at 10k), Platform (+20%, $18.00 at 10k). Overage per 1,000 extra: Basic $1.80 ($1.70 at >=300k), Pro $1.30 ($1.25), Platform $1.20 ($1.15). Monthly billing only (no annual plans offered).

Not published / not verified: Exact 100k and 500k prices (calculator has no stop at those volumes).

Figures verified 2026-08-29 on the Postmark (ActiveCampaign) pricing page, promo and intro discounts excluded — method and corrections via the claims methodology.

Price your own volume: 10,000 sends/month, 125,000, or 700,000.

Where Email Fast stands

Our stake, declared: we make Email Fast, and these are the launch prices already published on our pricing page — during early access they may be refined before general availability, and self-serve checkout opens at GA. Contacts and subscribers are never billed, at any list size.

Where Postmark wins

From our own pricing survey, their side stated plainly — Best-in-class transactional deliverability reputation and delivery speed (separate transactional/broadcast message streams), superb developer experience, and a free tier that never expires.

Honest additions: a transactional reputation earned over many years is not a thing a new platform can claim to match — ours has to be earned the same way. If Postmark isn't hurting you on price or missing a feature you need, staying is a defensible choice. The full comparison: Postmark vs Email Fast.

Next

• The step-by-step migration guide
• Also pricing SendGrid or Mailgun?
• Suspended by your current platform? Read that first.
• Apply for early access — live sending starts by application

Postmark is a trademark of its owner; Email Fast is not affiliated with or endorsed by them. Postmark facts were verified on their public pages on the dates shown — corrections: hello@emailfast.dev.