403 'You can only send testing emails to your own email address' (Resend)

Symptom: POST to the emails endpoint returns 403 with the message "You can only send testing emails to your own email address" and points you to resend.com/domains.

1. Confirm the cause: check the from address in your request. If it ends in the resend.dev test domain and any to/cc/bcc recipient is not your Resend account email, this restriction is the cause. It is not an API key problem.
2. Understand the rule: the resend.dev domain exists for testing only. It can only send to the email address on your Resend account, which protects the shared domain reputation.
3. Fix: go to the Domains page in the dashboard and click Add Domain. Enter a domain you own.
4. Copy the DNS records Resend shows and add them at your DNS provider. Wait for the domain to verify (usually within 15 minutes).
5. Update every API call: change the from address from the resend.dev test address to an address on your verified domain. Do not leave any test calls pointing at resend.dev.
6. Verify: send to a recipient that is not your own address and confirm you get a 200 with an email id instead of the 403.