Email (Resend)

lib/mail.ts exposes mailer and sendTransactionalEmail(), built on createMailer() / renderTransactionalEmail() from @flaredev/core, which call Resend’s REST API directly with fetch — no SDK dependency to bundle into your Worker.

Local development needs no account

With RESEND_API_KEY left empty — the scaffold default — emails print to the console instead of being sent, so sign-up, password reset, and any email flow you build work locally with zero setup. MAIL_FROM falls back to Resend’s sandbox sender, which only delivers to the account owner, so set your own verified domain before going further than local testing.

Retries are opt-in and idempotent

send() resolves to { data, error } and never throws for API errors — matching Resend’s own SDK contract. 429/5xx responses and idempotency conflicts are retried with backoff (honoring Retry-After), but only when you pass an idempotencyKey — without one, a retry could double-send.

import { sendTransactionalEmail } from "@/lib/mail";

await sendTransactionalEmail({
  to: user.email,
  subject: "Welcome to Acme",
  heading: "Welcome, " + user.name,
  paragraphs: ["Your account is ready."],
  action: { label: "Open dashboard", url: `${baseUrl}/dashboard` },
  footer: "You received this because you signed up for Acme.",
  idempotencyKey: `welcome-email/${user.id}`,
});

sendTransactionalEmail takes the fields of TransactionalEmail (from @flaredev/core) except appName, which lib/mail.ts fills in, plus to, subject and an optional idempotencyKey:

Field	Type
heading	string	Required
paragraphs	string[]	Required; plain text, HTML-escaped
action	{ label: string; url: string }	Optional button; url must be absolute http(s)
footer	string	Optional small print
preheader	string	Optional inbox preview text

The template

A monochrome, table-based email — the only layout that survives every email client’s CSS support gap — with escaped content, an optional action button (absolute http(s) URLs only), and a matching plain-text part generated alongside the HTML.

Setting it up for real

1. Verify a sending domain in Resend.

2. Set RESEND_API_KEY and MAIL_FROM (a verified address on that domain) — locally in .dev.vars, in production with:

   wrangler secret put RESEND_API_KEY
   wrangler secret put MAIL_FROM

flare deploy lists any optional secret from .dev.vars.example that’s still unset in production, RESEND_API_KEY included, so you won’t discover a silent no-op mailer after the fact.