Send email from Next.js with Mailbay

1. 1

   Verify your domain

   The from address must be on a domain verified in your Mailbay project. Records and checks are in the domains docs.
2. 2

   Create a Sending only key

   It can send and read the send log, and nothing else. Keep it in server env vars.
3. 3

   Set two env vars

   RESEND_BASE_URL and RESEND_API_KEY in .env.local and in your host's settings. Never with a NEXT_PUBLIC_ prefix.

import { Resend } from "resend";

const resend = new Resend(process.env.RESEND_API_KEY); // reads RESEND_BASE_URL

export async function POST(request: Request) {
  const { email } = await request.json();
  const { data, error } = await resend.emails.send({
    from: "Acme <noreply@acme.com>",
    to: [email],
    subject: "Welcome to Acme",
    html: "<p>Thanks for signing up.</p>",
  });
  if (error) return Response.json({ error }, { status: 400 });
  return Response.json(data); // { id: "eml_..." }
}

import { Mailbay, MailbayError } from "mailbay";

const mailbay = new Mailbay(process.env.MAILBAY_API_KEY);

export async function POST(request: Request) {
  const { email } = await request.json();
  try {
    const { id } = await mailbay.emails.send({
      from: "Acme <noreply@acme.com>",
      to: email,
      subject: "Welcome to Acme",
      html: "<p>Thanks for signing up.</p>",
    });
    return Response.json({ id });
  } catch (e) {
    if (e instanceof MailbayError) return Response.json({ code: e.code, fix: e.fix }, { status: e.status || 500 });
    throw e;
  }
}

export async function POST(request: Request) {
  const { email } = await request.json();
  const res = await fetch("https://api.mailbay.dev/v1/emails", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.MAILBAY_API_KEY}`,
      "Content-Type": "application/json",
      "Idempotency-Key": `welcome-${email}`, // a retry never sends twice
    },
    body: JSON.stringify({
      from: "Acme <noreply@acme.com>",
      to: email,
      subject: "Welcome to Acme",
      html: "<p>Thanks for signing up.</p>",
    }),
  });
  // { id } on success, { error: { code, message, fix } } otherwise
  return Response.json(await res.json(), { status: res.status });
}

The full request and response, scheduling and batches are in the Emails API docs; the SDK's methods and errors are in the Node SDK docs.

• POST /v1/emails

  Welcome and receipt emails

  From your own domain, with html, text, attachments and tags.

• Schedule and batch

  Send later with scheduled_at, or up to 100 emails in one request.

• Auth emails from Supabase

  Using Supabase Auth? Point it at Mailbay with custom SMTP, no code needed.

• Retries that never double send

  The same Idempotency-Key within 24 hours returns the first result.

• Know what happened

  Read a send's status, or get message.* webhooks for delivered and bounced.

On October 14, 2026 we created a Next.js 16.4.0 app with these route handlers, set .env.local to a local Mailbay stack, and posted to each one. Each returned an email id, and Mailbay's API reported both emails delivered: the capture at the top of this page. Before sending to real people from your domain, send one message to the email deliverability test.

403 sender_not_verified

The from address is not on a verified domain of the key's project. The error's fix field names the commands.

401 invalid_api_key

The key is missing or wrong. Check the variable name, and restart next dev after editing .env.local.

400 missing_body

Add text or html, or both. Every error says what to change.

The SDK still calls Resend

RESEND_BASE_URL is not set where the code runs. Moving from Resend? See the Resend alternative page.

Shipping a SaaS on Next.js? Mailbay for SaaS teams covers support inboxes and webhooks too. All other frameworks are in the integrations directory.