Resend

Resend is a developer-focused email API for sending transactional and marketing emails via REST or SDK.

Documentation

• Docs: https://resend.com/docs/llms.txt

Best Practices

Unsubscribe links are never auto-injected. Resend does not append unsubscribe headers or links automatically. You must manually include an unsubscribe link in every marketing email body, and configure a custom unsubscribe page separately. Omitting this violates CAN-SPAM/GDPR and causes deliverability issues.

The resend.dev sandbox domain only sends to your own verified email. Using onboarding@resend.dev as the from address will produce a 403 error if the recipient is anyone other than the account owner's email. Always switch to a verified domain before sending to real users.

Use idempotency keys on send to prevent duplicate emails on retry. Network failures trigger retries, and without an idempotency key each retry creates a new send. Pass a stable idempotency_key (e.g. derived from your database record ID) in the request header so Resend deduplicates server-side.