tools-email

name	tools-email
description	Email delivery using Resend API. Use this skill when implementing email verification flows, password reset, transactional emails, configuring DNS (SPF/DKIM/DMARC), setting up the Resend MCP server, or following email best practices for deliverability. Includes Next.js 16 proxy patterns, OAuth vs password user handling, and token security patterns.

Email Resend Skill

This skill provides workflows, best practices, and code patterns for sending transactional emails using Resend.

Overview

Resend is a modern email API designed for developers. It provides:

• Simple REST API and SDKs
• React Email integration for beautiful templates
• Built-in analytics and deliverability monitoring
• MCP server for AI-assisted email workflows

Prerequisites

1. Resend Account Setup

1. Sign up at resend.com
2. Generate an API key in the dashboard
3. Add to your environment:

# .env.local
RESEND_API_KEY=re_xxxxx
RESEND_FROM_EMAIL="App Name <[e-mail]>"

2. Domain Verification

Before sending from your domain, you must verify it:

1. Go to Resend Dashboard → Domains → Add Domain
2. Add these DNS records at your provider:

Type	Name	Value	Purpose
TXT	@ or domain	v=spf1 include:_spf.resend.com ~all	SPF
CNAME	resend._domainkey	Provided by Resend	DKIM
TXT	_dmarc	v=DMARC1; p=none; rua=mailto:[e-mail]	DMARC

3. Wait for verification (usually 5-60 minutes)

3. MCP Server Setup (Optional - for Cursor AI)

To use Resend directly from Cursor's AI:

1. Clone and build the MCP server:

cd ~/Desktop/Code
git clone https://github.com/resend/mcp-send-email.git mcp-send-email
cd mcp-send-email && npm install && npm run build

2. Get the absolute path (this is critical):

realpath ~/Desktop/Code/mcp-send-email/build/index.js
# Output: /Users/YOUR_USERNAME/Desktop/Code/mcp-send-email/build/index.js

3. Add to your .cursor/mcp.json:

{
  "mcpServers": {
    "resend": {
      "type": "command",
      "command": "node /Users/YOUR_USERNAME/Desktop/Code/mcp-send-email/build/index.js --key=re_xxxxx --sender=[e-mail]"
    }
  }
}

Common Gotcha: The path must be absolute and correct. ~/Code/ vs ~/Desktop/Code/ will cause MODULE_NOT_FOUND errors. Always verify with realpath.

Code Patterns

Basic Resend Client (TypeScript)

// lib/email/index.ts
import { Resend } from 'resend'

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

const FROM_EMAIL = process.env.RESEND_FROM_EMAIL || 'App <[e-mail]>'
const APP_URL = process.env.NEXT_PUBLIC_APP_URL || 'http://localhost:3000'

export interface SendEmailOptions {
  to: string
  subject: string
  html: string
  text?: string
  replyTo?: string
}

export async function sendEmail(options: SendEmailOptions) {
  const { data, error } = await resend.emails.send({
    from: FROM_EMAIL,
    to: options.to,
    subject: options.subject,
    html: options.html,
    text: options.text,
    replyTo: options.replyTo,
  })

  if (error) {
    console.error('[Email] Failed to send:', error)
    throw new Error(`Failed to send email: ${error.message}`)
  }

  return data
}

Verification Email

export async function sendVerificationEmail(email: string, token: string) {
  const verifyUrl = `${APP_URL}/verify-email?token=${token}`
  
  return sendEmail({
    to: email,
    subject: 'Verify your email address',
    html: `
      <h2>Welcome!</h2>
      <p>Please verify your email address by clicking the button below:</p>
      <a href="${verifyUrl}" style="
        display: inline-block;
        background: #000;
        color: #fff;
        padding: 12px 24px;
        text-decoration: none;
        border-radius: 6px;
        margin: 16px 0;
      ">Verify Email</a>
      <p>Or copy this link: ${verifyUrl}</p>
      <p>This link expires in 24 hours.</p>
      <p style="color: #666; font-size: 12px;">
        If you didn't create an account, you can ignore this email.
      </p>
    `,
    text: `Verify your email: ${verifyUrl}`,
  })
}

Password Reset Email

export async function sendPasswordResetEmail(email: string, token: string) {
  const resetUrl = `${APP_URL}/reset-password?token=${token}`
  
  return sendEmail({
    to: email,
    subject: 'Reset your password',
    html: `
      <h2>Password Reset Request</h2>
      <p>We received a request to reset your password. Click below to choose a new one:</p>
      <a href="${resetUrl}" style="
        display: inline-block;
        background: #000;
        color: #fff;
        padding: 12px 24px;
        text-decoration: none;
        border-radius: 6px;
        margin: 16px 0;
      ">Reset Password</a>
      <p>Or copy this link: ${resetUrl}</p>
      <p>This link expires in 1 hour.</p>
      <p style="color: #666; font-size: 12px;">
        If you didn't request this, you can safely ignore this email.
      </p>
    `,
    text: `Reset your password: ${resetUrl}`,
  })
}

Welcome Email (after verification)

export async function sendWelcomeEmail(email: string, name?: string) {
  return sendEmail({
    to: email,
    subject: 'Welcome to App Name!',
    html: `
      <h2>Welcome${name ? `, ${name}` : ''}!</h2>
      <p>Your email has been verified and your account is ready.</p>
      <p>Here are some things you can do:</p>
      <ul>
        <li>Complete your profile</li>
        <li>Explore features</li>
        <li>Check out the documentation</li>
      </ul>
      <a href="${APP_URL}" style="
        display: inline-block;
        background: #000;
        color: #fff;
        padding: 12px 24px;
        text-decoration: none;
        border-radius: 6px;
        margin: 16px 0;
      ">Get Started</a>
    `,
    text: `Welcome! Your account is ready. Get started at ${APP_URL}`,
  })
}

Database Schema

Add these tables for email verification and password reset:

-- Add to users table
ALTER TABLE users ADD COLUMN email_verified_at TEXT;

-- Email verification tokens (one-time use)
CREATE TABLE email_verification_tokens (
  id TEXT PRIMARY KEY,
  user_id TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
  token_hash TEXT NOT NULL UNIQUE,  -- SHA-256 hash, never store raw
  expires_at TEXT NOT NULL,
  created_at TEXT NOT NULL DEFAULT (datetime('now'))
);

-- Password reset tokens (one-time use)
CREATE TABLE password_reset_tokens (
  id TEXT PRIMARY KEY,
  email TEXT NOT NULL,  -- Use email, not user_id (user might not exist)
  token_hash TEXT NOT NULL UNIQUE,
  expires_at TEXT NOT NULL,
  created_at TEXT NOT NULL DEFAULT (datetime('now'))
);

CREATE INDEX idx_verification_tokens_hash ON email_verification_tokens(token_hash);
CREATE INDEX idx_reset_tokens_hash ON password_reset_tokens(token_hash);

Token Generation Pattern

Use secure, URL-safe tokens for verification and reset links:

// lib/auth/tokens.ts
import { sha256 } from 'oslo/crypto'
import { encodeBase64url, encodeHex } from 'oslo/encoding'

// Token valid for 24 hours
const VERIFICATION_TOKEN_EXPIRY = 24 * 60 * 60 * 1000
// Reset token valid for 1 hour  
const RESET_TOKEN_EXPIRY = 60 * 60 * 1000

/**
 * Generate a cryptographically secure token
 */
export function generateToken(): string {
  const bytes = new Uint8Array(32)
  crypto.getRandomValues(bytes)
  return encodeBase64url(bytes)
}

/**
 * Hash a token for database storage
 * Never store raw tokens - always hash them
 */
export async function hashToken(token: string): Promise<string> {
  return encodeHex(await sha256(new TextEncoder().encode(token)))
}

Token Security Rules

1. Never store raw tokens - Always hash with SHA-256 before storing
2. One-time use - Delete token after successful validation
3. Delete existing tokens - When creating new token, delete any existing ones for that user/email
4. Short expiry for reset - Password reset tokens should expire in 1 hour max
5. Longer expiry for verification - Email verification can be 24-48 hours

Rate Limiting

Prevent email abuse with rate limiting:

// Per-email rate limits
const EMAIL_RATE_LIMITS = {
  verification: { max: 3, window: 60 * 60 * 1000 },    // 3 per hour
  passwordReset: { max: 3, window: 60 * 60 * 1000 },   // 3 per hour
  general: { max: 10, window: 24 * 60 * 60 * 1000 },   // 10 per day
}

Resend API Limits

Plan	Emails/day	Emails/month	Rate limit
Free	100	3,000	2/second
Pro	5,000+	Based on plan	10/second

OAuth vs Password Users

When implementing email verification, handle OAuth and password users differently:

// OAuth users (Google, Apple, etc.) - pre-verified
const user = createUser({
  // ...
  email_verified_at: new Date().toISOString(), // Trust OAuth provider
})

// Password users - require verification
const user = createUser({
  // ...
  email_verified_at: null, // Must verify via email
})

Verification Flow by Auth Type

Auth Type	Email Verified?	Verification Required?
Google OAuth	Yes (by Google)	No
Apple OAuth	Yes (by Apple)	No
Email/Password	No	Yes - block until verified
Magic Link	Yes (implicit)	No

Handling Unverified Login Attempts

// In login API
if (!user.email_verified_at) {
  return NextResponse.json({
    error: 'Please verify your email before signing in.',
    code: 'EMAIL_NOT_VERIFIED',
    requiresVerification: true,
  }, { status: 403 })
}

Re-signup for Unverified Users

If a user tries to sign up with an email that exists but isn't verified, resend the verification:

const existingUser = getUserByEmail(email)
if (existingUser && !existingUser.email_verified_at) {
  // Resend verification instead of returning error
  const { token } = await createVerificationToken(existingUser.id)
  await sendVerificationEmail(email, token)
  return { requiresVerification: true }
}

Next.js Integration

Next.js 16+ (proxy.ts)

Breaking Change: Next.js 16 replaced middleware.ts with proxy.ts

For protected routes, use proxy.ts for fast redirects only:

// src/proxy.ts
export function proxy(request: NextRequest) {
  const { pathname } = request.nextUrl
  
  // Allow auth routes
  if (['/login', '/verify-email', '/forgot-password', '/reset-password']
      .some(r => pathname.startsWith(r))) {
    return NextResponse.next()
  }
  
  // Let API routes handle their own auth
  return NextResponse.next()
}

Important: Don't do heavy auth validation in proxy. Check email_verified_at in your API routes and server components instead.

Next.js 16 and earlier

Use middleware.ts with the same logic, but export as middleware instead of proxy.

Best Practices

Subject Lines

• Keep under 60 characters
• Be specific and action-oriented
• Avoid spam triggers (see references/deliverability.md)

Email Copy

• Front-load important information
• Use clear CTAs
• Always include plain text fallback
• Keep emails focused on one purpose

Transactional vs Marketing

• Transactional: Triggered by user action (verification, reset, receipts)
• Marketing: Promotional content (newsletters, announcements)
• Keep them separate - different sending reputations

Error Handling

• Log all email failures
• Have fallback mechanisms (show token in UI for dev)
• Don't block user actions on email failures

Verification Strategies

Strategy	UX	Security	Use Case
Block until verified	Friction	High	Financial, healthcare
Soft verification (banner)	Smooth	Medium	Social, content apps
No verification	Seamless	Low	Low-risk apps

Implementation Checklist

Email Verification Flow

• Add email_verified_at column to users table
• Create email_verification_tokens table
• Install resend and oslo packages
• Create email service (lib/email/index.ts)
• Create token utilities (lib/auth/tokens.ts)
• Modify signup to send verification email
• Create /api/auth/verify-email endpoint
• Create /api/auth/resend-verification endpoint (rate limited)
• Create /verify-email page with resend UI
• Update login to check email_verified_at
• Handle OAuth users as pre-verified

Password Reset Flow

• Create password_reset_tokens table
• Create /api/auth/forgot-password endpoint
• Create /api/auth/reset-password endpoint
• Create /forgot-password page
• Create /reset-password page
• Add "Forgot password?" link to login page

Environment Variables

RESEND_API_KEY=re_xxxxx
RESEND_FROM_EMAIL="App Name <[e-mail]>"
NEXT_PUBLIC_APP_URL=https://yourapp.com

References

• Deliverability Guide - DNS, spam prevention, reputation
• Email Templates - Copy best practices, compliance
• React Email Patterns - Component-based email templates

Common Issues & Troubleshooting

MCP Server "MODULE_NOT_FOUND"

Cause: Wrong path in .cursor/mcp.json

Fix: Use absolute path, verify with realpath:

realpath ~/Desktop/Code/mcp-send-email/build/index.js

Emails Going to Spam

1. Verify DNS records (SPF, DKIM, DMARC) are correct
2. Check sender domain matches authenticated domain
3. Review email content for spam trigger words
4. Test with mail-tester.com

Token Validation Failing

1. Ensure you're hashing the token before lookup
2. Check token hasn't expired
3. Verify token wasn't already consumed (one-time use)
4. Check for URL encoding issues in the token

OAuth Users Can't Reset Password

OAuth-only users don't have passwords. Check for password_hash:

if (!user.password_hash) {
  return { error: 'This account uses social login.' }
}

Next.js 16 Proxy Not Working

1. File must be src/proxy.ts (not middleware.ts)
2. Export must be proxy (not middleware)
3. Proxy runs on Node.js runtime, not Edge

Dependencies

Install the Resend SDK:

npm install resend

For token hashing (recommended):

npm install oslo

For React Email templates (optional):

npm install @react-email/components react-email