Send Email with Resend Skill

Send Email with Resend

Overview

Resend provides two endpoints for sending emails:

| Approach | Endpoint | Use Case | |----------|----------|----------| | Single | POST /emails | Individual transactional emails, emails with attachments, scheduled sends | | Batch | POST /emails/batch | Multiple distinct emails in one request (max 100), bulk notifications |

Choose batch when:

Sending 2+ distinct emails at once
Reducing API calls is important (by default, rate limit is 2 requests per second)
No attachments or scheduling needed

Choose single when:

Sending one email
Email needs attachments
Email needs to be scheduled
Different recipients need different timing

Quick Start

Detect project language from config files (package.json, requirements.txt, go.mod, etc.)
Install SDK (preferred) or use cURL - See references/installation.md
Choose single or batch based on the decision matrix above
Implement best practices - Idempotency keys, error handling, retries

Best Practices (Critical for Production)

Always implement these for production email sending. See references/best-practices.md for complete implementations.

Idempotency Keys

Prevent duplicate emails when retrying failed requests.

| Key Facts | | |-----------|---| | Format (single) | <event-type>/<entity-id> (e.g., welcome-email/user-123) | | Format (batch) | batch-<event-type>/<batch-id> (e.g., batch-orders/batch-456) | | Expiration | 24 hours | | Max length | 256 characters | | Duplicate payload | Returns original response without resending | | Different payload | Returns 409 error |

Error Handling

| Code | Action | |------|--------| | 400, 422 | Fix request parameters, don't retry | | 401, 403 | Check API key / verify domain, don't retry | | 409 | Idempotency conflict - use new key or fix payload | | 429 | Rate limited - retry with exponential backoff (by default, rate limit is 2 requests/second) | | 500 | Server error - retry with exponential backoff |

Retry Strategy

Backoff: Exponential (1s, 2s, 4s...)
Max retries: 3-5 for most use cases
Only retry: 429 (rate limit) and 500 (server error)
Always use: Idempotency keys when retrying

Single Email

Endpoint: POST /emails (prefer SDK over cURL)

Required Parameters

| Parameter | Type | Description | |-----------|------|-------------| | from | string | Sender address. Format: "Name <email@domain.com>" | | to | string[] | Recipient addresses (max 50) | | subject | string | Email subject line | | html or text | string | Email body content |

Optional Parameters

| Parameter | Type | Description | |-----------|------|-------------| | cc | string[] | CC recipients | | bcc | string[] | BCC recipients | | reply_to* | string[] | Reply-to addresses | | scheduled_at* | string | Schedule send time (ISO 8601) | | attachments | array | File attachments (max 40MB total) | | tags | array | Key/value pairs for tracking (see Tags) | | headers | object | Custom headers |

*Parameter naming varies by SDK (e.g., replyTo in Node.js, reply_to in Python).

Minimal Example (Node.js)

import { Resend } from 'resend';

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

const { data, error } = await resend.emails.send(
  {
    from: 'Acme <onboarding@resend.dev>',
    to: ['delivered@resend.dev'],
    subject: 'Hello World',
    html: '<p>Email body here</p>',
  },
  { idempotencyKey: `welcome-email/${userId}` }
);

if (error) {
  console.error('Failed:', error.message);
  return;
}
console.log('Sent:', data.id);

See references/single-email-examples.md for all SDK implementations with error handling and retry logic.

Batch Email

Endpoint: POST /emails/batch (but prefer SDK over cURL)

Limitations

No attachments - Use single sends for emails with attachments
No scheduling - Use single sends for scheduled emails
Atomic - If one email fails validation, the entire batch fails
Max 100 emails per request
Max 50 recipients per individual email in the batch

Pre-validation

Since the entire batch fails on any validation error, validate all emails before sending:

Check required fields (from, to, subject, html/text)
Validate email formats
Ensure batch size <= 100

Minimal Example (Node.js)

import { Resend } from 'resend';

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

const { data, error } = await resend.batch.send(
  [
    {
      from: 'Acme <notifications@acme.com>',
      to: ['delivered@resend.dev'],
      subject: 'Order Shipped',
      html: '<p>Your order has shipped!</p>',
    },
    {
      from: 'Acme <notifications@acme.com>',
      to: ['delivered@resend.dev'],
      subject: 'Order Confirmed',
      html: '<p>Your order is confirmed!</p>',
    },
  ],
  { idempotencyKey: `batch-orders/${batchId}` }
);

if (error) {
  console.error('Batch failed:', error.message);
  return;
}
console.log('Sent:', data.map(e => e.id));

See references/batch-email-examples.md for all SDK implementations with validation, error handling, and retry logic.

Large Batches (100+ Emails)

For sends larger than 100 emails, chunk into multiple batch requests:

Split into chunks of 100 emails each
Use unique idempotency keys per chunk: <batch-prefix>/chunk-<index>
Send chunks in parallel for better throughput
Track results per chunk to handle partial failures

See references/batch-email-examples.md for complete chunking implementations.

Deliverability

Follow these practices to maximize inbox placement.

For more help with deliverability, install the email-best-practices skill with npx skills add resend/email-best-practices.

Required

| Practice | Why | |----------|-----| | Valid SPF, DKIM, DMARC record | authenticate the email and prevent spoofing | | Links match sending domain | If sending from @acme.com, link to https://acme.com - mismatched domains trigger spam filters | | Include plain text version | Use both html and text parameters for accessibility and deliverability (Resend generates a plain text version if not provided) | | Avoid "no-reply" addresses | Use real addresses (e.g., support@) - improves trust signals | | Keep body under 102KB | Gmail clips larger messages |

Tracking (Opens & Clicks)

Tracking is configured at the domain level in the Resend dashboard, not per-email.

| Setting | How it works | Recommendation | |---------|--------------|----------------| | Open tracking | Inserts 1x1 transparent pixel | Disable for transactional emails - can hurt deliverability | | Click tracking | Rewrites links through redirect | Disable for sensitive emails (password resets, security alerts) |

When to enable tracking:

Marketing emails where engagement metrics matter
Newsletters and announcements

When to disable tracking:

Transactional emails (receipts, confirmations, password resets)
Security-sensitive emails
When maximizing deliverability is priority

Configure via dashboard: Domain → Configuration → Click/Open Tracking

Webhooks (Event Notifications)

Track email delivery status in real-time using webhooks. Resend sends HTTP POST requests to your endpoint when events occur.

| Event | When to use | |-------|-------------| | email.delivered | Confirm successful delivery | | email.bounced | Remove from mailing list, alert user | | email.complained | Unsubscribe user (spam complaint) | | email.opened / email.clicked | Track engagement (marketing only) |

CRITICAL: Always verify webhook signatures. Without verification, attackers can send fake events to your endpoint.

See references/webhooks.md for setup, signature verification code, and all event types.

Templates

Use pre-built templates instead of sending HTML with each request.

const { data, error } = await resend.emails.send({
  from: 'Acme <hello@acme.com>',
  to: ['delivered@resend.dev'],
  subject: 'Welcome!',
  template: {
    id: 'tmpl_abc123',
    variables: {
      USER_NAME: 'John',      // Case-sensitive!
      ORDER_TOTAL: '$99.00'
    }
  }
});

IMPORTANT: Variable names are case-sensitive and must match exactly as defined in the template editor. USER_NAME ≠ user_name.

| Fact | Detail | |------|--------| | Max variables | 20 per template | | Reserved names | FIRST_NAME, LAST_NAME, EMAIL, RESEND_UNSUBSCRIBE_URL, contact, this | | Fallback values | Optional - if not set and variable missing, send fails | | Can't combine with | html, text, or react parameters |

Templates must be published in the dashboard before use. Draft templates won't work.

Testing

WARNING: Never test with fake addresses at real email providers.

Using addresses like test@gmail.com, example@outlook.com, or fake@yahoo.com will:

Bounce - These addresses don't exist
Destroy your sender reputation - High bounce rates trigger spam filters
Get your domain blocklisted - Providers flag domains with high bounce rates

Safe Testing Options

| Method | Address | Result | |--------|---------|--------| | Delivered | delivered@resend.dev | Simulates successful delivery | | Bounced | bounced@resend.dev | Simulates hard bounce | | Complained | complained@resend.dev | Simulates spam complaint | | Your own email | Your actual address | Real delivery test |

For development: Use the resend.dev test addresses to simulate different scenarios without affecting your reputation.

For staging: Send to real addresses you control (team members, test accounts you own).

Domain Warm-up

New domains must gradually increase sending volume to establish reputation.

Why it matters: Sudden high volume from a new domain triggers spam filters. ISPs expect gradual growth.

Recommended Schedule

Existing domain

| Day | Messages per day | Messages per hour | |-----|---------------------|---------------------| | 1 | Up to 1,000 emails | 100 Maximum | | 2 | Up to 2,500 emails | 300 Maximum | | 3 | Up to 5,000 emails | 600 Maximum | | 4 | Up to 5,000 emails | 800 Maximum | | 5 | Up to 7,500 emails | 1,000 Maximum | | 6 | Up to 7,500 emails | 1,500 Maximum | | 7 | Up to 10,000 emails | 2,000 Maximum |

New domain

| Day | Messages per day | Messages per hour | |-----|---------------------|---------------------| | 1 | Up to 150 emails | | | 2 | Up to 250 emails | | | 3 | Up to 400 emails | | | 4 | Up to 700 emails | 50 Maximum | | 5 | Up to 1,000 emails | 75 Maximum | | 6 | Up to 1,500 emails | 100 Maximum | | 7 | Up to 2,000 emails | 150 Maximum |

Monitor These Metrics

| Metric | Target | Action if exceeded | |--------|--------|-------------------| | Bounce rate | < 4% | Slow down, clean list | | Spam complaint rate | < 0.08% | Slow down, review content |

Don't use third-party warm-up services. Focus on sending relevant content to real, engaged recipients.

Suppression List

Resend automatically manages a suppression list of addresses that should not receive emails.

Addresses are added when:

Email hard bounces (address doesn't exist)
Recipient marks email as spam
You manually add them via dashboard

What happens: Resend won't attempt delivery to suppressed addresses. The email.suppressed webhook event fires instead.

Why this matters: Continuing to send to bounced/complained addresses destroys your reputation. The suppression list protects you automatically.

Management: View and manage suppressed addresses in the Resend dashboard under Suppressions.

Common Mistakes

| Mistake | Fix | |---------|-----| | Retrying without idempotency key | Always include idempotency key - prevents duplicate sends on retry | | Using batch for emails with attachments | Batch doesn't support attachments - use single sends instead | | Not validating batch before send | Validate all emails first - one invalid email fails the entire batch | | Retrying 400/422 errors | These are validation errors - fix the request, don't retry | | Same idempotency key, different payload | Returns 409 error - use unique key per unique email content | | Tracking enabled for transactional emails | Disable open/click tracking for password resets, receipts - hurts deliverability | | Using "no-reply" sender address | Use real address like support@ - improves trust signals with email providers | | Not verifying webhook signatures | Always verify - attackers can send fake events to your endpoint | | Testing with fake emails (test@gmail.com) | Use delivered@resend.dev - fake addresses bounce and hurt reputation | | Template variable name mismatch | Variable names are case-sensitive - USER_NAME ≠ user_name | | Sending high volume from new domain | Warm up gradually - sudden spikes trigger spam filters |

Notes

The from address must use a verified domain
If the sending address cannot receive replies, set the reply_to parameter to a valid address.
Store API key in RESEND_API_KEY environment variable
Node.js SDK supports react parameter for React Email components
Resend returns error, data, headers in the response.
Data returns { id: "email-id" } on success (single) or array of IDs (batch)
For marketing campaigns to large lists, use Resend Broadcasts instead

Agent Skills: Send Email with Resend

Install this agent skill to your local

Skill Files