Email with Resend
Setting up transactional email, and the SPF/DKIM/DMARC deliverability warning.
Magic-code sign-in emails and any receipt or notification email you add all go out through Resend
in production. Without a Resend key, development uses letter_opener instead, opening every email
in your browser rather than sending it anywhere, so nothing about local development requires this
setup.
The two variables
RESEND_API_KEY, from resend.com.MAIL_FROM, a sender address on a domain you've verified inside Resend.
Deliverability: the step that actually matters
Add Resend's SPF, DKIM, and DMARC DNS records for your sending domain. Skip this, and magic-code emails will land in spam, or fail to deliver at all, which silently breaks signup for anyone affected: they never see an error, they just never get their code. This is the single most common cause of "sign-in doesn't work" reports on a freshly deployed app, and it's invisible from inside the app itself, since the email leaves your server successfully; it just doesn't arrive anywhere useful. Set these DNS records up at the same time you're configuring the rest of your domain; see Edge and DNS Setup.
Next
Turn on real Stripe billing: Stripe Setup. Or continue through the reference: Sign in with Google and Apple.