Anything your project sends by email, a form or booking notification on a Website, a signup confirmation or password reset on a Web Application, an organization invite, or a Send Email block on an API project, goes out through YOUR OWN Resend account, not a shared account CodeBlock Studio runs for every project. Until you connect one, that email simply cannot be sent, and whatever triggered it shows a clear message explaining that, rather than silently doing nothing. The Website and Web Application notification emails described here are available on every plan, including Free, the API project's own Send Email block requires a Plus or Creator plan, see "Free, Plus, and Creator plans compared."
Connecting your account
Open your project's Integrations tab (for an API project, its Keys and Secrets tab instead) and paste a Resend API key together with a from address. The from address must be on a domain you've verified inside Resend, Resend itself will not deliver mail from an unverified domain. Once saved, the key itself is never shown again, only a confirmation that it's connected.
What breaks without it
Most places this affects, form and booking notifications on a Website, sign in codes and reset links on a Web Application, fail quietly and simply skip the email, since these already have a working fallback (a submission is still saved, a reset link request always responds the same way either way). A directly triggered action, like a Send Email block or an organization invite, instead returns a real error explaining that the app's owner needs to connect email sending first.
Tip: One Resend account and one from address covers every email feature in a single project. You don't need a separate connection per feature.