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

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