# Resend Email Service Integration Standard > **Scope:** universal > **Layer:** 2 (on keyword) > **Keywords:** resend, email, transactional, smtp, sendgrid alternative > **Load When:** resend or transactional email keywords detected **Verified against:** Resend Node SDK 4 + Resend .NET SDK + React Email. Last-verified: 2026-05-20. --- Modern transactional email service for developers, built for Next.js and .NET. --- ## Overview Resend provides: - Simple API for sending emails - React Email template support - Email tracking and analytics - Domain verification - Webhooks for delivery status - 100 free emails/day **Stack:** Universal (.NET + Next.js) --- ## Core Principles 1. **API-First**: Use REST API, not SMTP 2. **Type Safety**: Use official SDKs with TypeScript/C# types 3. **Template-Based**: Use React Email for Next.js, Razor for .NET 4. **Async**: Always send emails asynchronously 5. **Monitor**: Track delivery status via webhooks --- ## Installation & Setup ### Next.js Setup ```bash npm install resend react-email ``` ```typescript // lib/resend.ts import { Resend } from 'resend'; export const resend = new Resend(process.env.RESEND_API_KEY); ``` ### .NET Setup ```bash dotnet add package Resend ``` ```csharp // Program.cs using Resend; builder.Services.AddOptions(); builder.Services.AddHttpClient(); builder.Services.Configure(o => { o.ApiToken = builder.Configuration["Resend:ApiKey"]!; }); builder.Services.AddTransient(); ``` ### Environment Variables ```bash # .env.local (Next.js) or appsettings.json (.NET) RESEND_API_KEY=re_... RESEND_FROM_EMAIL=noreply@yourdomain.com ``` --- ## Next.js Usage ### React Email Template ```tsx // emails/WelcomeEmail.tsx import { Html, Button, Container, Text } from '@react-email/components'; interface WelcomeEmailProps { name: string; verificationUrl: string; } export default function WelcomeEmail({ name, verificationUrl }: WelcomeEmailProps) { return ( Welcome, {name}! ); } ``` ### Send Email (API Route) ```typescript // app/api/send-welcome/route.ts import { resend } from '@/lib/resend'; import WelcomeEmail from '@/emails/WelcomeEmail'; export async function POST(req: Request) { const { email, name, verificationUrl } = await req.json(); try { const { data, error } = await resend.emails.send({ from: 'MyApp ', to: email, subject: 'Welcome to MyApp!', react: WelcomeEmail({ name, verificationUrl }), }); if (error) { return Response.json({ error }, { status: 500 }); } return Response.json({ id: data.id }); } catch (error) { return Response.json({ error: 'Failed to send email' }, { status: 500 }); } } ``` --- ## .NET Usage ### Email Service Interface ```csharp // Services/IEmailService.cs public interface IEmailService { Task SendWelcomeEmailAsync(string toEmail, string name); Task SendPasswordResetAsync(string toEmail, string resetUrl); } ``` ### Email Service Implementation ```csharp // Services/EmailService.cs using Resend; public class EmailService : IEmailService { private readonly IResend _resend; private readonly IConfiguration _config; public EmailService(IResend resend, IConfiguration config) { _resend = resend; _config = config; } public async Task SendWelcomeEmailAsync(string toEmail, string name) { var message = new EmailMessage { From = _config["Resend:FromEmail"]!, To = toEmail, Subject = "Welcome!", HtmlBody = $@"

Welcome, {name}!

Thank you for joining us.

" }; var response = await _resend.EmailSendAsync(message); return response.Id; } public async Task SendPasswordResetAsync(string toEmail, string resetUrl) { var message = new EmailMessage { From = _config["Resend:FromEmail"]!, To = toEmail, Subject = "Reset Your Password", HtmlBody = $@"

Password Reset

Click the link below to reset your password:

Reset Password " }; var response = await _resend.EmailSendAsync(message); return response.Id; } } // Register in DI builder.Services.AddScoped(); ``` --- ## Batch Emails ### Next.js Batch Send ```typescript // Send to multiple recipients const { data, error } = await resend.batch.send([ { from: 'MyApp ', to: 'user1@example.com', subject: 'Notification', react: NotificationEmail({ message: 'Update 1' }), }, { from: 'MyApp ', to: 'user2@example.com', subject: 'Notification', react: NotificationEmail({ message: 'Update 2' }), }, ]); ``` --- ## Webhooks for Delivery Status ### Setup Webhook Endpoint Resend webhooks are signed with **Svix** — verify with `resend.webhooks.verify()`, which checks the `svix-id` / `svix-timestamp` / `svix-signature` headers against the raw request body. Do **not** roll a manual HMAC: Resend does not use a plain `resend-signature` header, and a hand-written check will reject valid webhooks. ```typescript // app/api/webhooks/resend/route.ts import { resend } from '@/lib/resend'; export async function POST(req: Request) { // Raw body is required — verification fails on a re-serialized payload const payload = await req.text(); let event; try { // Throws on an invalid signature; returns the parsed payload on success event = resend.webhooks.verify({ payload, headers: { id: req.headers.get('svix-id')!, timestamp: req.headers.get('svix-timestamp')!, signature: req.headers.get('svix-signature')!, }, webhookSecret: process.env.RESEND_WEBHOOK_SECRET!, }); } catch { return new Response('Invalid signature', { status: 401 }); } switch (event.type) { case 'email.delivered': await handleEmailDelivered(event.data); break; case 'email.bounced': await handleEmailBounced(event.data); break; case 'email.complained': await handleEmailComplained(event.data); break; } return new Response('OK'); } ``` --- ## Best Practices ### Background Jobs (with Hangfire) ```csharp // Don't block HTTP requests with email sending BackgroundJob.Enqueue( x => x.SendWelcomeEmailAsync(email, name)); ``` ### Rate Limiting ```typescript // Implement rate limiting for email sends import { Ratelimit } from '@upstash/ratelimit'; import { Redis } from '@upstash/redis'; const ratelimit = new Ratelimit({ redis: Redis.fromEnv(), limiter: Ratelimit.slidingWindow(10, '1 h'), // 10 emails per hour }); const { success } = await ratelimit.limit(userId); if (!success) { return Response.json({ error: 'Rate limit exceeded' }, { status: 429 }); } await resend.emails.send({ ... }); ``` ### Retry Logic (.NET) ```csharp using Polly; var retryPolicy = Policy .Handle() .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))); await retryPolicy.ExecuteAsync(async () => { await _resend.EmailSendAsync(message); }); ``` --- ## Domain Verification 1. Go to Resend Dashboard → Domains 2. Add your domain (e.g., `yourdomain.com`) 3. Add DNS records (SPF, DKIM, DMARC) 4. Wait for verification (up to 72 hours) ### DNS Records Example ``` TXT @ v=spf1 include:resend.com ~all TXT resend._domainkey TXT _dmarc v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com ``` --- ## Common Email Templates ### Order Confirmation ```tsx export default function OrderConfirmation({ orderNumber, total, items }) { return ( Order #{orderNumber} Confirmed Total: ${total}
    {items.map(item => (
  • {item.name} - ${item.price}
    • ))}
); } ``` ### Magic Link Auth ```tsx export default function MagicLink({ url, expiresIn }) { return ( Sign in to your account This link expires in {expiresIn} minutes. ); } ``` --- ## References - [Resend Documentation](https://resend.com/docs) - [React Email](https://react.email/) - [Resend .NET SDK](https://github.com/resend/resend-dotnet) --- *MORPH-SPEC by Polymorphism Tech*