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

1. Detect project language from config files (package.json, requirements.txt, go.mod, etc.)
2. Install SDK (preferred) or use cURL - See references/installation.md
3. Choose single or batch based on the decision matrix above
4. 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:

1. Split into chunks of 100 emails each
2. Use unique idempotency keys per chunk: <batch-prefix>/chunk-<index>
3. Send chunks in parallel for better throughput
4. 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

Recommended

Practice	Why
Use subdomains	Send transactional from notifications.acme.com, marketing from mail.acme.com - protects reputation
Disable tracking for transactional	Open/click tracking can trigger spam filters for password resets, receipts, etc.

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.

Tags

Tags are key/value pairs that help you track and filter emails.

tags: [
  { name: 'user_id', value: 'usr_123' },
  { name: 'email_type', value: 'welcome' },
  { name: 'plan', value: 'enterprise' }
]

Use cases:

• Associate emails with customers in your system
• Categorize by email type (welcome, receipt, password-reset)
• Filter emails in the Resend dashboard
• Correlate webhook events back to your application

Constraints: Tag names and values can only contain ASCII letters, numbers, underscores, or dashes. Max 256 characters each.

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