Resend API notes

What the Resend adapter does under the hood — the endpoints, the limits, the quirks worth knowing, and the capabilities it declares.

Capabilities

Capability	Value	Consequence
`custom_headers`	true	Our headers are passed through.
`injects_unsubscribe_headers`	false	Nothing is auto-injected, so the RFC 8058 headers are entirely ours.
`send_time_suppression`	true	Silent, region-wide drops for bounces and complaints.
`suppression_read`	`:none`	No API at all. The nightly suppression sync doesn't run.
`suppression_webhooks`	true	The only route for Resend-side suppression.
`engagement_webhooks`	false	Per-domain tracking stays off — tracking is ours.
`push_unsubscribe`	false	No write API. The standing guard covers it.
`remote_delete`	false	GDPR erase can't remove the address Resend-side.

Sending

Case	Endpoint
One message	`POST /emails`
More than one	`POST /emails/batch`, chunked at 100

Rate limit: 10 requests/second, team-wide. The adapter declares emails_per_minute: 600, and the sender's pacer counts every send against that budget — one message per request through the queue, so 600 a minute is the honest ceiling. The account-wide maximum (sending.max_emails_per_minute, 300 by default) applies on top: the effective pace is whichever of the two is lower.

Headers

"headers": {
  "List-Unsubscribe": "<https://your-mimeo.com/u/SIGNED_TOKEN>",
  "List-Unsubscribe-Post": "List-Unsubscribe=One-Click"
}

Resend injects nothing, so these are ours. The token is the same signed token the subscription page takes, which is what attributes a header unsubscribe to the exact email it came from. Both halves matter: without List-Unsubscribe-Post, a mail client shows a link rather than its own unsubscribe button.

The mail client's POST /u/:token goes directly to Mimeo and globally unsubscribes the person. It does not first pass through Resend or depend on a Resend webhook. Hosted subscription preferences do not change this behavior; selective tag or field changes use the separate preference form.

Transactional mail carries no unsubscribe header. You can't opt out of your own password reset.

Webhooks

Resend event	Normalizes to
`email.delivered`	`delivered`
`email.bounced`	`bounced`
`email.complained`	`complained`
`email.suppressed`	`suppressed`
anything else	acknowledged and ignored

A bounce's type maps to our bounce kind: Permanent → hard (suppresses the person), Transient → soft (doesn't — a full mailbox is a try-again, not a dead address).

Signed with Svix: HMAC-SHA256 over svix-id.svix-timestamp.body, base64, compared in constant time, with a five-minute freshness window so a captured request can't be replayed tomorrow. Multiple v1,… signatures are accepted so a secret can be rotated without dropping anything mid-flight. See Webhooks for the shared contract.

Quirks

The User-Agent header is mandatory. Resend answers 403 without one, which is a confusing way to learn you forgot it.
Idempotency keys expire after 24 hours. Fine — they exist to make one retry of one send safe, not to deduplicate forever.
The batch endpoint drops attachments. Mimeo doesn't use attachments.
DKIM is 1024-bit only.
Accepted ≠ delivered. A suppressed address gets a success-shaped response and no email.

Credentials

Setting	What
`provider.resend.api_key`	The API key.
`provider.resend.webhook_secret`	The Svix signing secret, `whsec_…`.

Both are entered on the Settings page and encrypted in your Mimeo's database, and neither ever appears in a manager repo.

verify_config calls GET /domains and fails when no domain is verified, naming the ones still pending — reporting a good connection that would refuse every send isn't a verification.