Mailgun migration guide

Migrate from Mailgun to an EU-hosted email API

Postscale keeps the developer shape familiar while combining transactional sending, inbound routing, masked aliases, DMARC reporting, and warming under one account.

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Migration scope

Convert Messages API calls from multipart form data to JSON.
Replace Routes with aliases, inbound rules, and webhooks.
Move bounces, complaints, and unsubscribes before production traffic.
Use SMTP relay where legacy systems cannot switch to REST immediately.

What changes when you move

Mailgun	Postscale	Migration note
Messages API	POST /v1/send	Map html/text fields to html_body/text_body and move o:tag values to tags.
Routes	Aliases and inbound rules	Use per-address aliases plus domain-level rules for sender, subject, and authentication conditions.
Webhooks	Postscale webhooks	Create one endpoint with selected event types and verify HMAC signatures.
Template Storage	Postscale templates	Import Handlebars-style HTML and send using template slug or ID.
Suppressions	Suppression API	Export bounces, complaints, and unsubscribes separately so reasons remain auditable.

Cut over in controlled steps

Mirror DNS

Add Postscale verification, DKIM, return-path, and optional MX records while the old provider keeps sending.

Convert code

Swap API credentials and normalize the send payload. Keep the old provider path behind a feature flag for rollback.

Move state

Import suppressions and template HTML, then configure delivery, bounce, complaint, and inbound webhooks.

Ramp traffic

Start at 10 percent, watch bounces and complaints by ISP, then increase volume as warming limits allow.

Send call conversion

Mailgun

const form = new FormData();
form.append("from", "Acme <alerts@example.com>");
form.append("to", "user@example.com");
form.append("subject", "Welcome");
form.append("html", "<p>Hello from Acme</p>");
form.append("o:tag", "onboarding");
form.append("v:user_id", "usr_123");

await fetch("https://api.mailgun.net/v3/example.com/messages", {
  method: "POST",
  headers: { Authorization: `Basic ${mailgunToken}` },
  body: form,
});

Postscale

await fetch("https://api.postscale.io/v1/send", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.POSTSCALE_API_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    from: "Acme <alerts@example.com>",
    to: ["user@example.com"],
    subject: "Welcome",
    html_body: "<p>Hello from Acme</p>",
    tags: ["onboarding"],
    metadata: { user_id: "usr_123" },
  }),
});

Production checklist

Lower DNS TTL to 300 seconds at least 24 hours before cutover.

Keep both DKIM selectors published during the transition.

Import hard bounces, complaints, unsubscribes, and manual suppressions before the first production send.

Run webhook handlers in dual-shape mode until all old-provider events have drained.

Send seed tests to Gmail, Outlook, Yahoo, iCloud, and a corporate Microsoft 365 tenant.

Cut over early in the week and avoid final traffic moves on Friday.

Monitor delivery, bounce, deferred, and complaint rates by ISP after each traffic increase.

Keep the old account active until delayed events and user replies have stopped arriving.

Frequently asked questions

What replaces Mailgun Routes?

Use Postscale aliases for address-level routing and inbound rules for conditional routing by sender, subject, headers, authentication results, or target webhook.

Can I keep SMTP during the migration?

Yes. Postscale supports SMTP relay, so legacy apps can change host, username, and password before you rewrite REST calls.

How should suppressions move from Mailgun?

Export bounces, complaints, unsubscribes, and manual suppressions as separate CSV files, dedupe them, then import before sending through Postscale.