Switching from Resend

Keep the same core send fields while moving authentication, delivery events, and inbound mail to PostShiba.

Move from Resend to PostShiba one slice of traffic at a time, with Resend ready as a fallback until you are sure. The core send fields, from, to, subject, and html, stay the same. The endpoint, the re_ API key, webhooks, and receiving change.

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.

• Your Resend API keys, verified domains, and sending subdomains.
• Every HTTP and SMTP caller, including how it uses idempotency keys, tags, attachments, and templates.
• Webhook URLs, their secrets, the events your code handles, and anything that depends on opens or clicks.
• Receiving domains, MX records, and attachment handling.
• Any audience or broadcast workflow. PostShiba does not replace these.

How Resend 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 separate teams or keys keep customers apart, create a tenant for each. 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 Resend Sending access key. It can call POST /api/v1/emails, the cluster's POST /sends, and GET /api/v1/users/me. Every other request returns 403. It is not tied to one domain. It can send from any verified domain on your team. If your app used a Full access re_ 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 Resend'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 Resend's DKIM, SPF, and bounce records in place. You need them if you roll back.

If Resend 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 bounced, complained, and unsubscribed addresses before the first real send. Every address you miss is a bounce or complaint on your new cluster.

1. Export those addresses from Resend or from your own database.
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. This example leaves tenant_id out, so the rows go into the default tenant. Add tenant_id to import into another tenant.

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

Switch one send path

The four core fields keep their names. If your Resend calls use idempotency keys, send on the cluster's POST /sends, which accepts the same Idempotency-Key header and replays the first response for 24 hours.

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.

A few more changes:

• Tags. Move tags into unique_args, up to 32 keys of plain values. See Unique args.
• Attachments. Fetch remote files yourself. Resend's path and content_id have no equivalent. Send filename, base64 content, and an optional content_type. See Attachments.
• Templates. Port Resend templates to PostShiba templates, or render HTML in your app.
• Volume. The REST send API allows 300 requests a minute per API key. For more than that, use SMTP or HTTP inject.

SMTP. Replace the Resend SMTP login with your PostShiba SMTP credential, on port 587 with STARTTLS and AUTH PLAIN. Put tracking fields in the X-Capsule-Unique-Args header as a JSON object. See Send emails with SMTP.

Replace delivery webhooks

1. Create a delivery webhook and store its secret.
2. Parse a JSON array and handle each object by its event field. Deduplicate on sg_event_id.
3. Replace the Svix signature check with PostShiba's. See Verify webhook requests.
4. Remove anything that waits for opens or clicks.
5. Read send errors such as suppressed from the send response, not from a webhook. See Errors.

The full field list is on Event types.

Bring your dedicated IPs

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

Resend dedicated IPs map to a PostShiba IP pool. You pick the pool per message with the X-Capsule-Pool header.

1. Count how many dedicated IPs 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 stream, 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>. For example, mail that used a Resend dedicated IP for 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 tenants 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 Resend 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. For stage 1, you can add "sandbox": true to a POST /sends body. It runs every check without sending. See Sending.

Move inbound mail

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

1. Create an inbox for each address. For catch-all receiving, create a domain-bound inbox with the local part *.
2. Use the receiving webhook. It carries the message content, so you do not need follow-up calls to fetch it. To poll instead, see Managing received emails.
3. Test on a hosted inbox or a new subdomain first.
4. Record the current MX record, lower its TTL, and wait for the old TTL to pass.
5. Point the MX record at the PostShiba inbound host shown on the inbox. Never leave both providers' MX records at the same priority. Replace any resend.app address in Reply-To.

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 Resend 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 the PostShiba webhook running so late events still arrive.
• Inbound. Restore the MX record you recorded. Keep both inbound handlers running until the old TTL has passed, because some senders cache MX records.

Keep Resend keys, webhooks, 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. Revoke your Resend API keys.
2. Remove Resend webhooks.
3. Delete Resend's DNS records. Keep the PostShiba DKIM, return-path, and MX records.

Final checklist

• The cluster is sending_ready and production approval is complete.
• Your API key, verified domains, and any tenants are in place.
• Suppressions are imported.
• Callers that retry use POST /sends with an Idempotency-Key.
• Tags, attachments, and templates have replacements.
• Both delivery and receiving webhooks verify PostShiba signatures over the raw body.
• SMTP callers use the PostShiba SMTP settings.
• Your router sends each message through exactly one provider.
• Dedicated IPs are in pools and finished their 14-day warm-up.
• Resend stays ready until the rollback window closes.

Next steps

• Sending
• Attachments
• Managing webhooks
• Receiving emails
• Set up IP pools and warm up dedicated IPs