Resend

Choose Send Email with Resend under Actions → Notifications. Enter a Resend API key, an address on a verified sending domain, and the sender name if wanted. Configure To, Reply-To, subject, and text or HTML message. Recipient and message settings accept form variables; the API key and sender address remain administrator-controlled.

The action sends a stable idempotency key and records Resend's email ID. Retries of the same saved action/submission reuse the key. Resend retains idempotency keys for 24 hours; this is not an unlimited remote exactly-once guarantee. Core Forms also prevents replay of an execution it has already recorded as completed.

Delivery and bounce events

Save the action, then copy its displayed webhook URL into Resend. Enable the required events, such as email.delivered, email.bounced, email.complained, email.failed, email.delivery_delayed, and email.suppressed. Paste that endpoint's whsec_ signing secret back into the action and save again.

The endpoint is /wp-json/cf/v1/resend/FORM_ID/ACTION_ID. It verifies the original request body against the Svix ID, timestamp, and signature, with a five-minute timestamp tolerance. It matches the email ID to the original form/action before recording delivery status. Duplicate event IDs are ignored. Recent events arriving before the send record is saved receive a retryable response; unrelated older email IDs are ignored.

Accepted sends and subsequent delivery events are separate action-log entries. Bounces, complaints, failures, and suppressions appear as failed resend_delivery entries. Recipient addresses and email content are not copied into delivery-event metadata. Events can arrive out of order; inspect their individual statuses instead of treating the last arriving event as a definitive progression.

Removing the action or its signing secret disables the endpoint for future events. Rotating the secret requires updating both sides. With no signing secret, email sending still works but delivery callbacks are unavailable.

References: send API, idempotency, webhook verification.