Email Delivery Skill

Comprehensive patterns and templates for implementing robust email delivery with Resend, covering single emails, batch operations, scheduled delivery, and attachment handling.

Use When

Building transactional email systems (confirmations, notifications, alerts)
Implementing batch email campaigns (up to 100 recipients per request)
Setting up scheduled/delayed email delivery
Handling file attachments, buffers, or URL-based attachments
Adding reply-to, CC, and BCC functionality
Creating email templates with variables and dynamic content
Implementing retry logic and delivery error handling

Core Patterns

1. Single Email Sending

Transactional emails for immediate delivery with minimal latency:

import { Resend } from 'resend';

const resend = new Resend('your_resend_key_here');

async function sendTransactionalEmail() {
  const { data, error } = await resend.emails.send({
    from: 'notifications@example.com',
    to: 'user@example.com',
    subject: 'Welcome to Example',
    html: '<h1>Welcome!</h1><p>Thank you for signing up.</p>',
  });

  if (error) {
    console.error('Failed to send email:', error);
    return null;
  }

  return data;
}

2. Batch Email Sending

Bulk operations for sending up to 100 emails in a single request:

async function sendBatchEmails(recipients: Array<{email: string; name: string}>) {
  const emails = recipients.map(recipient => ({
    from: 'newsletter@example.com',
    to: recipient.email,
    subject: `Hello ${recipient.name}!`,
    html: `<p>Welcome ${recipient.name}</p>`,
  }));

  const { data, error } = await resend.batch.send(emails);

  if (error) {
    console.error('Batch send failed:', error);
    return null;
  }

  return data;
}

3. Scheduled Email Delivery

Time-based delivery for emails sent at specific times:

async function scheduleEmail(scheduledAt: Date) {
  const { data, error } = await resend.emails.send({
    from: 'marketing@example.com',
    to: 'user@example.com',
    subject: 'Scheduled Message',
    html: '<p>This was scheduled!</p>',
    scheduled_at: scheduledAt.toISOString(),
  });

  if (error) {
    console.error('Failed to schedule email:', error);
    return null;
  }

  return data;
}

4. Attachment Handling

File attachments from files, buffers, or URLs:

File-based Attachment

import fs from 'fs';
import path from 'path';

async function sendWithFileAttachment(filePath: string) {
  const fileContent = fs.readFileSync(filePath);
  const fileName = path.basename(filePath);

  const { data, error } = await resend.emails.send({
    from: 'documents@example.com',
    to: 'recipient@example.com',
    subject: 'Your Document',
    html: '<p>Please find attached your document.</p>',
    attachments: [
      {
        filename: fileName,
        content: fileContent,
      },
    ],
  });

  return { data, error };
}

Buffer-based Attachment

async function sendWithBufferAttachment(buffer: Buffer, filename: string) {
  const { data, error } = await resend.emails.send({
    from: 'reports@example.com',
    to: 'user@example.com',
    subject: 'Monthly Report',
    html: '<p>Your monthly report is attached.</p>',
    attachments: [
      {
        filename: filename,
        content: buffer,
      },
    ],
  });

  return { data, error };
}

URL-based Attachment

async function sendWithUrlAttachment(fileUrl: string) {
  const response = await fetch(fileUrl);
  const buffer = await response.arrayBuffer();

  const { data, error } = await resend.emails.send({
    from: 'notifications@example.com',
    to: 'user@example.com',
    subject: 'Download Your File',
    html: '<p>Your file is ready.</p>',
    attachments: [
      {
        filename: 'document.pdf',
        content: Buffer.from(buffer),
      },
    ],
  });

  return { data, error };
}

5. Reply-To and CC/BCC

Message routing with multiple recipients and reply addresses:

async function sendWithRouting(mainRecipient: string) {
  const { data, error } = await resend.emails.send({
    from: 'support@example.com',
    to: mainRecipient,
    reply_to: 'support-team@example.com',
    cc: ['manager@example.com'],
    bcc: ['archive@example.com'],
    subject: 'Support Ticket #12345',
    html: '<p>We received your support request.</p>',
  });

  return { data, error };
}

Python Patterns

Single Email (Python)

import os
from resend import Resend

client = Resend(api_key=os.environ.get("RESEND_API_KEY"))

def send_email():
    email = {
        "from": "notifications@example.com",
        "to": "user@example.com",
        "subject": "Welcome",
        "html": "<h1>Welcome!</h1>",
    }

    response = client.emails.send(email)
    return response

Batch Email (Python)

def send_batch_emails(recipients):
    emails = [
        {
            "from": "newsletter@example.com",
            "to": recipient["email"],
            "subject": f"Hello {recipient['name']}",
            "html": f"<p>Welcome {recipient['name']}</p>",
        }
        for recipient in recipients
    ]

    response = client.batch.send(emails)
    return response

File Attachment (Python)

def send_with_attachment(file_path):
    with open(file_path, 'rb') as f:
        file_content = f.read()

    email = {
        "from": "documents@example.com",
        "to": "recipient@example.com",
        "subject": "Your Document",
        "html": "<p>Document attached.</p>",
        "attachments": [
            {
                "filename": "document.pdf",
                "content": file_content,
            }
        ],
    }

    response = client.emails.send(email)
    return response

Template Variables

Environment Variables Required

RESEND_API_KEY=your_resend_key_here
RESEND_FROM_EMAIL=your-verified-email@example.com

Email Template Variables

interface EmailPayload {
  from: string;              // Verified sender email
  to: string | string[];     // Recipient(s)
  cc?: string[];            // Carbon copy recipients
  bcc?: string[];           // Blind carbon copy
  reply_to?: string;        // Reply-to address
  subject: string;          // Email subject
  html?: string;            // HTML content
  text?: string;            // Plain text fallback
  attachments?: Array<{
    filename: string;
    content: Buffer | string;
  }>;
  scheduled_at?: string;    // ISO 8601 datetime for scheduling
  tags?: Array<{
    name: string;
    value: string;
  }>;
}

Best Practices

Error Handling

Always implement retry logic for transient failures:

async function sendWithRetry(emailPayload, maxRetries = 3) {
  for (let attempt = 1; attempt <= maxRetries; attempt++) {
    const { data, error } = await resend.emails.send(emailPayload);

    if (!error) return { data, success: true };

    if (error.message?.includes('rate_limit') && attempt < maxRetries) {
      const delay = Math.pow(2, attempt) * 1000;
      await new Promise(resolve => setTimeout(resolve, delay));
      continue;
    }

    return { error, success: false };
  }
}

Rate Limiting

Resend has rate limits. For batch operations over 100 emails:

async function sendLargeBatch(emails: EmailPayload[]) {
  const batchSize = 100;
  const results = [];

  for (let i = 0; i < emails.length; i += batchSize) {
    const batch = emails.slice(i, i + batchSize);
    const { data, error } = await resend.batch.send(batch);

    if (error) {
      console.error(`Batch ${Math.floor(i / batchSize) + 1} failed:`, error);
      results.push({ success: false, error });
    } else {
      results.push({ success: true, data });
    }

    // Rate limit handling - wait between batches
    if (i + batchSize < emails.length) {
      await new Promise(resolve => setTimeout(resolve, 1000));
    }
  }

  return results;
}

Template Best Practices

Use responsive HTML templates
Always provide text fallback
Test across email clients
Include unsubscribe links for marketing emails
Avoid large attachments (keep under 25MB)

Examples Directory Structure

single-email/ - Basic transactional email patterns
batch-emails/ - Bulk sending with 50-100 recipients
attachments/ - File, buffer, and URL attachment handling
scheduled/ - Time-based delivery scheduling

See individual example README files for complete code and usage patterns.

Related Skills

email-templates - HTML template management and rendering
email-validation - Recipient address validation
email-webhooks - Delivery event tracking and bounce handling

Resources

Security Notes

API keys should be stored in environment variables (never hardcoded)
Use RESEND_API_KEY from secure secret management
Verify sender emails before using in production
Implement authentication for email submission endpoints
Log email events for compliance and debugging

email-delivery