Postmark vs Mailchimp Transactional

1. Inventory Postmark-specific behavior

List every Server, Message Stream, sender signature or domain, template alias/ID, template model, tag, metadata field, attachment, suppression rule, and webhook consumer. Record which application behavior depends on each field before changing code.

2. Replace authentication and map the send payload

Map Postmark's From, To, Subject, HtmlBody, TextBody, Tag, and Metadata fields into Mailchimp Transactional's message object. Do not carry the Postmark server token into the new client.

const mailchimp = require("@mailchimp/mailchimp_transactional")(
  process.env.MAILCHIMP_TRANSACTIONAL_API_KEY
);

await mailchimp.messages.send({
  message: {
    from_email: "receipts@example.com",
    to: [{ email: customer.email, type: "to" }],
    subject: "Your receipt",
    html: receiptHtml,
    text: receiptText,
    tags: ["receipt"],
    metadata: { order_id: order.id },
  },
});

3. Rebuild templates instead of copying IDs

Postmark TemplateId, TemplateAlias, and TemplateModel are provider-specific. Recreate each template in Mailchimp Transactional, translate variables and conditional syntax, then switch template sends to messages.sendTemplate. Render golden test cases for missing, empty, escaped, and non-ASCII values.

4. Translate events, suppressions, and domain setup

Map Postmark delivery, bounce, spam-complaint, subscription-change, and open/click payloads into your internal event model before replacing webhooks. Treat suppressions as customer-consent and deliverability state: preserve required unsubscribe and complaint records, but do not blindly import stale addresses. Complete Mailchimp Transactional sender-domain verification and DKIM before traffic moves.

5. Canary, reconcile, then cut over

Route a deterministic small cohort through exactly one provider. Compare accepted, delivered, bounced, complained, latency, template output, and webhook reconciliation. Increase traffic only after the new path passes, keep Postmark available for rollback, and retire its credentials after the rollback window closes.