Resend 403 on first send: the from address must belong to a verified domain

The from-address rules that 403 you

Two knowledge-base pages cover this, and together they explain almost every first-send 403:

1. The test address only sends to yourself. Emails from the default test address can only go to your own email address. Sending to anyone else gets a 403 telling you to verify a domain and change the from address.

2. Any other from domain must be verified. If the domain in your from address does not match a verified domain on your account, the API returns a 403 domain-mismatch error.

Checklist

• Before writing any send code, verify the domain in the dashboard and set the from address to an address on it. Doing this after the code is written turns a two-minute setup step into a debugging session.

• When a send 403s, read the error: test-address-to-stranger vs unverified domain are different fixes. The error text names which one.

• The to address matters too during testing: with the test from address, keep recipients to your own address until the domain is verified.

• Deliverability starts here as well: a verified domain with proper DNS is what keeps the email out of spam.

Resend's knowledge base documents the test-address recipient restriction and the verified-domain requirement behind first-send 403s. Verified live at https://resend.com/docs/knowledge-base/403-error-resend-dev-domain and https://resend.com/docs/knowledge-base/403-error-domain-mismatch.