Switching from Mailchimp Transactional

Keep your transactional application flow while you replace Mandrill auth, templates, webhooks, and inbound routes with PostShiba.

Move from Mailchimp Transactional, formerly Mandrill, to PostShiba one slice of traffic at a time, with Mandrill ready as a fallback until you are sure. Your app's flow stays the same. The API key, request shape, templates, webhooks, and inbound routes change. This guide covers the Transactional API and SMTP service, not Mailchimp Marketing audiences or campaigns.

Before you start

• A PostShiba team with a payment method. Request production approval early, because you can only send to your own team until it is approved.
• Access to DNS for each sending and receiving domain.
• An HTTPS endpoint that can receive webhooks.

Take an inventory

Write these down before you change anything.

• Every caller of /messages/send, /messages/send-template, and SMTP, with where its key is stored and any X-MC-* headers it sets.
• Subaccounts, with their domains and traffic.
• Stored templates, their merge language, and the merge vars they use.
• Metadata, recipient_metadata, and tags, and the code that reads them later.
• Webhook URLs, the events they subscribe to, their keys, and your mandrill_events parser.
• Inbound domains, routes, MX records, and TTLs.

Keep a recent sample of messages and events so you can compare results during the move.

How Mandrill concepts map to PostShiba

Set up PostShiba

1. Create a cluster. Choose transactional or bulk to match the mail you are moving, and size the plan for your busiest hour at full volume.
2. Create a Send only API key for each app and environment, and store it on your server.
3. If you use subaccounts, create a tenant for each before adding its domains. Otherwise use the default tenant. See Setting up PostShiba for multi-tenant apps.
4. Add each sending domain under its tenant.
5. Issue an SMTP credential on the cluster for each extra tenant. The default tenant gets one when you create the cluster. REST sends need a live credential for the tenant too.
6. Wait until the cluster is sending_ready and approval is complete before sending real traffic.

Keep the key safe

A Send only key is PostShiba's version of a Mandrill API key limited to messages/send and messages/send-template. It can call POST /api/v1/emails, including template sends, the cluster's POST /sends, and GET /api/v1/users/me. Every other request returns 403. If your app used an unrestricted Mandrill key, the migration is a good time to drop to Send only.

Adding domains, creating webhooks, and importing suppressions need a Full access key. Use it from a script or the CLI, and do not deploy it with your app. Never put either key in browser or mobile code. A leaked Send only key can still send mail from your verified domains, so delete it on API keys if it leaks.

Publish DNS records

PostShiba uses its own DKIM selector and return-path host, so its records sit next to Mandrill's.

1. Open the domain in PostShiba and copy the records it shows.

2. Publish them at your DNS provider.

   Record	Host	Type
   DKIM	ps1._domainkey.mail.example.com	CNAME
   Return-path	rp.mail.example.com	CNAME

3. Click Check DNS and wait for the domain to verify.

4. Leave Mandrill's DKIM, SPF, and return-path records in place. You need them if you roll back.

If Mandrill already uses ps1._domainkey or rp. on that domain, the records collide. Send from a new subdomain on PostShiba instead. If a record will not verify, see What if my domain is not verifying?.

Bring your suppressions

Import your Mandrill rejects and unsubscribes before the first real send. Every address you miss is a bounce or complaint on your new cluster.

1. Export the rejection denylist with /rejects/list, for each subaccount. Add any unsubscribe list you keep in your app.
2. Import each list into the matching tenant. Upload the CSV on the Suppressions page, or call the import API.

Each request takes up to 10,000 addresses, and a CSV upload can be up to 2 MB. Imported rows have the reason Manual. Leave tenant_id out to import into the default tenant.

While both providers run, write new unsubscribes and complaints to a store that both send paths check.

Switch one send path

Change one caller at a time.

• Remove key from the body and send it as a Bearer token.
• Combine message.from_name and message.from_email into one from string.
• Split message.to by its type into to, cc, and bcc, keeping display names.
• Move headers["Reply-To"] into reply_to.
• Move metadata and tags into unique_args.
• Send one request per recipient when the content differs per person.

A 201 response has queued: true and a message_id. Store the message_id with the provider name, so you never resend a message PostShiba already accepted. To match delivery events to your own records, put your own id in unique_args. It comes back on every event.

Templates. PostShiba templates use Liquid, not Mandrill merge tags. Rewrite *|FNAME|* as {{ name }}, publish the template, and send with template.id and variables. See Send with a template. You can also keep rendering text and html in your app.

A few more changes:

• Tags. Turn each tag into a plain field, such as message_type. unique_args takes up to 32 keys. Never put secrets or message content in it. See Unique args.
• Tenants. Use a tenant slug or id you have confirmed. An unknown tenant is rejected. See Errors.
• Retries. If your retries need to be safe, send on the cluster's POST /sends with an Idempotency-Key header. See Sending.
• Volume. The REST send API allows 300 requests a minute per API key. For more than that, use SMTP or HTTP inject.

SMTP. Replace smtp.mandrillapp.com and the API key password with your PostShiba SMTP credential, on port 587 with STARTTLS and AUTH PLAIN. Replace X-MC-Metadata and X-MC-Tags with the X-Capsule-Unique-Args header. The credential decides the tenant, so drop X-MC-Subaccount. See Send emails with SMTP.

Replace delivery webhooks

1. Create a delivery webhook and store its secret. Update your receiver before you move any traffic.
2. Parse a JSON array of events. The body is raw JSON, not a form field named mandrill_events.
3. Verify the raw body before you parse it. See Verify webhook requests.
4. Deduplicate on sg_event_id.
5. Remove anything that waits for opens or clicks.

The full field list is on Event types.

Bring your dedicated IPs

Skip this section if you send on Mandrill's shared IPs.

Mailchimp Transactional dedicated IPs and the ip_pool send parameter map to a PostShiba IP pool. You pick the pool per message with the X-Capsule-Pool header.

1. Count how many dedicated IPs and pools you use, and which streams use each. Keep transactional and bulk apart.
2. Buy the same number of dedicated IPs in PostShiba. Create one pool per Mandrill pool, using the same name where you can. Add the IPs, then attach each pool to the matching cluster.
3. Send each stream with X-Capsule-Pool: <name> where you used to set ip_pool. For example, ip_pool: "transactional" becomes the header X-Capsule-Pool: transactional.
4. Do this at least 14 days before stage 2, so the parallel run below doubles as the warm-up.

The IP addresses themselves change, so update any allowlists your recipients keep. See Set up IP pools and warm up dedicated IPs for the full schedule and how to watch it.

Run both providers in parallel

Route each new message to exactly one provider. Pick a stable slice, such as a set of recipients or one message type, and record which provider sent each message. Retries and rollback then stay on the same provider. Never send the same message through both.

Adjust the schedule for your volume and risk. Keep Mandrill as primary while you ramp. Raise PostShiba's share only while deferrals stay down at Gmail, Yahoo, and Outlook. On shared IPs, this ramp tests your integration and builds your domain's reputation with mailbox providers. If you send through an IP pool, the same ramp is your IP warm-up, so start the pool's IPs at least 14 days before stage 2 and check the per-provider numbers before each stage.

Move inbound mail

Move receiving separately, after outbound is stable. MX records cannot split mail by percentage.

1. Replace each Mandrill route with a domain-bound inbox for that address, or one domain-bound catch-all inbox with the local part *. Move any other matching into your app.
2. Pick the json or sendgrid_parse webhook format. Neither posts mandrill_events, so update your handler. See Receiving webhooks.
3. Test on a hosted inbox or a new subdomain first.
4. Lower the TTL on the current MX record, and wait for the old TTL to pass.
5. If Mandrill receives on the same host you want PostShiba to use, replace its MX record with the one shown on the inbox. Otherwise point your Reply-To addresses at the new PostShiba address.

Receive on a subdomain, not on a host that already has mailboxes. See How do I avoid MX conflicts?.

Roll back

• Outbound. Point your router back at Mandrill for new messages only. Do not resend anything that already has a PostShiba message_id, because it was accepted and may still be delivered. Keep both webhook handlers running so late events still arrive. Match any unclear sends by your ids in unique_args.
• Inbound. Restore Mandrill's MX record. Keep both inbound handlers running until the old TTL has passed, because some senders cache MX records.

Keep Mandrill keys, webhooks, inbound routes, and DNS records until your rollback window closes.

Finish the cutover

After you have run at 100% for your whole rollback window with healthy checks:

1. Delete your Mandrill API keys.
2. Remove Mandrill webhooks and inbound routes.
3. Delete Mandrill's DNS records. Keep the PostShiba DKIM, return-path, and MX records.

Final checklist

• The cluster is sending_ready and production approval is complete.
• Every tenant that sends has a verified domain and a live credential.
• Rejects and unsubscribes are imported into the right tenants.
• Templates are ported to Liquid or rendered in your app.
• API callers use a Bearer token, and SMTP callers use the PostShiba SMTP settings.
• Metadata and tags arrive as plain unique_args.
• Delivery events verify and update the right records.
• Inbound mail and its webhook work before the MX change.
• Your router sends each message through exactly one provider.
• Dedicated IPs are in pools and finished their 14-day warm-up.
• Mandrill stays ready until the rollback window closes.

Next steps

• Send emails with HTTP
• Send with a template
• Managing webhooks
• Receiving emails
• Set up IP pools and warm up dedicated IPs