Integrating Resend for Transactional Email Sending

Integrating Resend for Transactional Email Sending

We have encountered projects where transactional emails never reached users: SMTP servers blocked them, DKIM was not configured, and templates had to be coded in raw HTML. As a result, lead forms, confirmations, and password resets were lost — the business took a hit. Losses from undelivered emails reached 15% conversion, and customers switched to competitors. Resend solves these problems out of the box: a modern email API with native React Email support, a free tier of 3000 emails per month, and automated deliverability setup. In this article we'll show how we connect Resend in real projects and what results it yields.

Why Resend instead of SMTP or legacy services?

Manual SMTP configuration means N+1 attempts, low deliverability, spam filters. Resend offers a ready API that guarantees delivery. Unlike SendGrid and Amazon SES, Resend is simpler: no need to manually set up DKIM/SPF — everything is automatic. Plus, React Email works out of the box, speeding up template development by orders of magnitude. Our experience shows: integration time is cut in half compared to classic solutions, and revenue from restored notifications pays for itself in 1-2 months.

Resend is 2x faster to set up than classic SMTP.

How we integrate Resend into your application

We connect Resend, configure your domain, develop React Email templates, set up batch sending and logging. We use TypeScript and Next.js 14 with Server Actions for sending from server components.

Installation and first send

npm install resend

import { Resend } from 'resend';

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

// Simple send
const { data, error } = await resend.emails.send({
  from: 'Acme <[email protected]>',
  to: ['[email protected]'],
  subject: 'Welcome to Acme!',
  html: '<h1>Hello!</h1><p>Thanks for registering.</p>',
});

if (error) {
  console.error('Email error:', error);
}

With React Email templates

import { Resend } from 'resend';
import { render } from '@react-email/render';
import WelcomeEmail from './emails/WelcomeEmail';

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

export async function sendWelcomeEmail(user: { email: string; name: string }) {
  const html = render(
    <WelcomeEmail name={user.name} loginUrl="https://app.acme.com/login" />
  );

  await resend.emails.send({
    from: 'Acme Team <[email protected]>',
    to: user.email,
    subject: 'Welcome to Acme!',
    html,
  });
}

How does batch sending work?

Batch sending allows you to send up to 100 emails in a single HTTP request. This is critical for invoice mailings, notifications, or confirmations during peak load. Resend processes them in parallel, saving time and resources.

await resend.batch.send([
  {
    from: '[email protected]',
    to: '[email protected]',
    subject: 'Your invoice is ready',
    html: invoiceHtml1,
  },
  {
    from: '[email protected]',
    to: '[email protected]',
    subject: 'Your invoice is ready',
    html: invoiceHtml2,
  },
]);

How to configure a domain for Resend (step by step)

1. Add your domain in the Resend Dashboard → Domains.
2. Add DNS records: SPF (TXT), DKIM (TXT × 3), Return-Path (MX).
3. Wait for verification (usually under 5 minutes).

After verification, you can use from: '[email protected]' instead of @resend.dev. Resend documentation states: "With proper DKIM and SPF setup, deliverability exceeds 99%."

How to avoid landing in spam?

Use a dedicated sending domain, set up a DMARC record, and monitor IP reputation through webhooks. Regularly clean inactive users. Resend automatically manages reputation, but basic list hygiene matters.

Webhooks for status tracking

Resend sends webhooks when email status changes. Below is a full Node.js handler example with Express:

app.post('/api/webhooks/resend', express.raw({ type: 'application/json' }), async (req, res) => {
  const signature = req.headers['resend-signature'];
  const event = JSON.parse(req.body);

  switch (event.type) {
    case 'email.sent':
      await db.emailLogs.update({ emailId: event.data.email_id, status: 'sent' });
      break;
    case 'email.delivered':
      await db.emailLogs.update({ emailId: event.data.email_id, status: 'delivered' });
      break;
    case 'email.bounced':
      await db.users.markEmailBounced(event.data.to[0]);
      break;
    case 'email.complained':
      await db.users.unsubscribe(event.data.to[0]);
      break;
  }

  res.status(200).end();
});

Common issues and solutions

• Authorization error: wrong API key. Check the RESEND_API_KEY environment variable.
• Emails not delivered: incorrect domain setup or landing in spam. Use webhooks for tracking and configure SPF/DKIM strictly per instructions.
• Rate limit exceeded: the free tier is capped at 3000 emails per month. If needed, upgrade to a paid plan or use batch sending to optimize.

What the Resend integration includes

Timeline and pricing

Basic Resend integration with 2-3 React Email templates takes 1-2 days. Extended version with domain, webhooks, and logging takes 3-4 days. Pricing is calculated individually based on project complexity. Leave a request — we will find the optimal solution. Get a consultation on Resend integration: our engineers with over 5 years of experience will help you set up reliable email sending. We guarantee high deliverability and post-launch support.

Email Campaign Integration: Why Does It Often Break?

We’ve observed that a trigger email sent 10 minutes after registration converts 4–5 times better than the same email sent after 24 hours. This isn’t a marketing myth—it’s mechanics: while the user is still warm, while they remember the context. But most integrations with email services are built like this: form submits → synchronous HTTP request to API → if the API is slow, the user waits 3 seconds → the email either goes out or doesn’t, nobody knows. In one project, we saw a 30% drop in conversion simply because the email service responded with 504 and Laravel’s queue driver wasn’t configured. Lost emails often hit customers silently – no log, no alert, just a missing order confirmation.

If you’re facing lost emails or spam folder issues, order an audit of your current integration – we’ll find bottlenecks within 2 days.

Providers and Their APIs

Unisender — a Russian provider popular in the SMB segment. REST API, simple. Adding a contact: importContacts, sending a transactional email: sendEmail. Important: for transactional emails (order confirmations, password resets), Unisender Go is a separate service with a different API and separate pricing. Mixing bulk and transactional mailings in one stream is bad for domain reputation. Unisender Go handles up to 1000 requests per second.

SendPulse — provides email, SMS, web push, Viber, and Telegram bots through a unified API. Convenient for projects requiring an omnichannel approach. Automation 360 is a visual chain builder; you can trigger automation via API events. The PHP SDK (sendpulse/rest-api-php-sdk) is maintained but updated irregularly – better to use Guzzle directly.

Mailchimp — a choice for international audiences and marketing teams accustomed to the Mailchimp ecosystem. Transactional email via Mandrill (a subsidiary service). Marketing API v3 for list, tag, and campaign management. Webhooks for opens, clicks, unsubscribes, bounces.

SMS. For Russia: SMSCenter, MTS Exolve, Devino Telecom, SMS Aero. Their APIs are similar: a send method with phone, message, sender parameters (sender name must be registered separately with the operator). One nuance: the sender name must be registered through the aggregator with a contract – otherwise SMS won’t be sent on MTS/MegaFon/Beeline networks.

How to Build an Integration That Doesn’t Lose Emails?

Separate Transactional and Marketing Streams

Transactional emails (order confirmations, password resets, delivery status) go through a dedicated sender domain or subdomain tx.example.com. Marketing campaigns go through mail.example.com or news.example.com. If a marketing campaign receives many spam complaints, it should not affect the reputation of the transactional stream. According to SendGrid documentation, transactional messages should be sent through a dedicated IP pool to prevent cross-contamination.

Queue and Retry

Any call to the email API goes through a queue (Laravel Queue, Bull, Celery). If Unisender returns a 503, the job retries after 5 minutes, then 15, then 60. After 5 failed attempts, it goes to a dead letter queue with an alert. The user already received their 200 OK and knows nothing about the issue. This approach reduces bounce rate on projects to 0.5%.

Example Laravel job:

public function handle(): void
{
    try {
        $response = Http::post(config('services.unisender.email_url'), $this->params);
        if ($response->failed()) {
            $this->release(300); // retry after 5 min
        }
    } catch (\Throwable $e) {
        $this->release(300);
    }
}

Templates

We store templates in code (Blade, Twig, React Email), not in the provider’s interface. Reasons: versioning via Git, browser preview without sending, testability. For complex templates with dynamic content — react-email with export to HTML via @react-email/render.

Validation and Consent

Before adding a contact to a list — double opt-in (confirmation email). Store the confirmation timestamp in your own database. Upon unsubscription — synchronously unsubscribe both at the provider and in your database. Ignoring webhook unsubscriptions is a direct path to account suspension at the provider. All processes comply with Федеральный закон № 152-ФЗ «О персональных данных».

Deliverability Monitoring and DKIM Setup

Connect provider webhooks for events: bounce (hard and soft), spam_complaint, unsubscribe. Hard bounce — immediately mark the email as invalid in your database, stop sending. Soft bounce 3 times in a row — same. Metrics: open rate, click rate, bounce rate, unsubscribe rate — review at least once a week. Our certified engineers configure alerts in Grafana/Prometheus.

DKIM configuration steps:

1. Generate a key pair (e.g., openssl genrsa -out private.key 2048).
2. Publish the public key in DNS as a TXT record for the selector (e.g., mail._domainkey.tx.example.com).
3. Provide the selector to the provider (SendGrid, Mailgun, Unisender).
4. Verify with dig TXT mail._domainkey.tx.example.com.

SPF, DKIM, DMARC must be configured separately for each stream. We use subdomains with different DNS records.

Why Is It Important to Separate Streams?

If you send a marketing campaign from the same domain as transactional emails and receive spam complaints, you risk getting the domain blocked — and users will stop receiving even order confirmations. SPF, DKIM, DMARC (Sender Policy Framework, DomainKeys Identified Mail, Domain‑based Message Authentication, Reporting and Conformance) must be configured separately for each stream. In one project, a marketing blast with 12% spam complaints blocked the transactional domain for 48 hours — we had to re‑authenticate with Google and Yandex.

What Does the Integration Scope Include?

• Audit of current communication streams and domain reputation (SPF, DKIM, DMARC)
• Provider and schema selection: transactional vs marketing traffic
• Configuration of SPF, DKIM, DMARC DNS records
• Development of email templates (HTML + dynamic content)
• Backend integration via queues and API
• Webhook setup for deliverability and complaints
• Operations documentation and team training
• Deliverability guarantee and post‑launch support

We deliver production‑ready documentation, access to monitoring dashboards, and a handover session with your engineers. Our certified engineers provide a 30‑day post‑launch health check guarantee.

Timelines and Cost

Cost is calculated individually after audit. We provide turnkey service: from analysis to production monitoring. Contact us for a free engineer consultation — we’ll evaluate your project and give accurate timelines. Over 7 years of experience in email service integration, 50+ projects implemented. Order a free audit of your current integration and receive a report with recommendations and estimated savings.