Send Batch Emails

Learn how to send batch emails efficiently with the Transactional TypeScript SDK. Includes error handling and rate limiting strategies.

Intermediate

15 minutes

batch

performance

bulk

Prerequisites

Node.js 18+ installed
Transactional account with API key
Verified sending domain
Basic understanding of async/await

CODE

Full Example

import { Transactional } from '@transactional/sdk';

const client = new Transactional({
  apiKey: process.env.TRANSACTIONAL_API_KEY!,
});

interface Recipient {
  email: string;
  name: string;
  data: Record<string, unknown>;
}

async function sendBatchEmails(recipients: Recipient[]) {
  const emails = recipients.map((recipient) => ({
    from: 'hello@yourdomain.com',
    to: recipient.email,
    subject: `Hello, ${recipient.name}!`,
    html: `
      <h1>Welcome, ${recipient.name}!</h1>
      <p>Thank you for joining our platform.</p>
    `,
    text: `Welcome, ${recipient.name}! Thank you for joining our platform.`,
    metadata: {
      userId: recipient.data.userId,
      campaign: 'welcome-batch',
    },
  }));
  const { data, error } = await client.emails.sendBatch({
    emails,
  });
  if (error) {
    console.error('Batch send failed:', error.message);
    return { success: false, error };
  }
  console.log(`Batch sent: ${data.successful} succeeded, ${data.failed} failed`);
  for (const failure of data.failures) {
    console.error(`Failed to send to ${failure.email}: ${failure.reason}`);
  }
  return { success: true, data };
}

const recipients: Recipient[] = [
  { email: 'user1@example.com', name: 'Alice', data: { userId: '1' } },
  { email: 'user2@example.com', name: 'Bob', data: { userId: '2' } },
  { email: 'user3@example.com', name: 'Charlie', data: { userId: '3' } },
];

sendBatchEmails(recipients);

Understanding Batch Sending

Batch sending allows you to send multiple emails in a single API request, which is more efficient than individual sends for bulk operations.

When to Use Batch Sending

Sending notifications to multiple users about the same event
Welcome email campaigns
Shipping notifications for multiple orders
Weekly digest emails
Any scenario with 10+ emails to send

Limits and Considerations

Limit	Value
Max emails per batch	100
Max request size	10 MB
Rate limit	1,000 emails/minute

Batch Response Handling

The batch send response provides detailed results:

interface BatchSendResult {
  successful: number;      // Count of successfully queued emails
  failed: number;          // Count of failed emails
  results: {
    email: string;
    id?: string;           // Message ID if successful
    status: 'queued' | 'failed';
    error?: string;        // Error message if failed
  }[];
  failures: {
    email: string;
    reason: string;
  }[];
}

Handling Partial Failures

Always check for partial failures in batch sends:

const { data, error } = await client.emails.sendBatch({ emails });
 
if (error) {
  // Complete failure - no emails were processed
  console.error('Batch request failed:', error.message);
  return;
}
 
// Check for partial failures
if (data.failed > 0) {
  console.log(`${data.failed} emails failed out of ${data.successful + data.failed}`);
 
  for (const failure of data.failures) {
    // Log for retry or investigation
    await logFailedEmail(failure.email, failure.reason);
  }
}

Retry Strategy for Failures

Implement exponential backoff for transient failures:

async function sendWithRetry(
  emails: EmailPayload[],
  maxRetries = 3
): Promise<BatchSendResult> {
  let lastError: Error | null = null;
 
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    const { data, error } = await client.emails.sendBatch({ emails });
 
    if (!error) {
      return data;
    }
 
    // Check if error is retryable
    if (error.code === 'RATE_LIMITED' || error.code === 'SERVICE_UNAVAILABLE') {
      const delay = Math.pow(2, attempt) * 1000; // 1s, 2s, 4s
      console.log(`Attempt ${attempt + 1} failed, retrying in ${delay}ms...`);
      await new Promise((resolve) => setTimeout(resolve, delay));
      lastError = new Error(error.message);
    } else {
      // Non-retryable error
      throw new Error(error.message);
    }
  }
 
  throw lastError || new Error('Max retries exceeded');
}

Performance Tips

1. Use Templates for Repeated Content

When sending similar emails to many recipients, use server-side templates to reduce payload size:

// Instead of sending full HTML for each email...
const emails = recipients.map(r => ({
  to: r.email,
  templateId: 'welcome-email',  // Much smaller payload
  templateData: { name: r.name },
}));

2. Process in Parallel with Limits

For very large sends, process batches in parallel but limit concurrency:

import pLimit from 'p-limit';
 
const limit = pLimit(5); // Max 5 concurrent batch requests
 
const chunks = chunkArray(allRecipients, 100);
const results = await Promise.all(
  chunks.map(chunk =>
    limit(() => client.emails.sendBatch({ emails: buildEmails(chunk) }))
  )
);

3. Stream Large Datasets

For millions of emails, stream from your database rather than loading all into memory:

async function* getRecipientsStream(cursor?: string) {
  while (true) {
    const batch = await db.users.findMany({
      take: 1000,
      cursor: cursor ? { id: cursor } : undefined,
    });
 
    if (batch.length === 0) break;
 
    yield batch;
    cursor = batch[batch.length - 1].id;
  }
}
 
for await (const recipientBatch of getRecipientsStream()) {
  await sendBatchEmails(recipientBatch);
}

Sources & References

[1]Node.js Streams Documentation — Node.js Foundation
[2]MDN Web Docs - Promise.allSettled() — Mozilla
[3]RFC 5321 - Simple Mail Transfer Protocol — Internet Engineering Task Force

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