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.
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.