Using Multiple SMTP Transporters in Production

2025-03-21 · Juan David

Many production applications need more than one email provider. You might send transactional emails through one service and marketing emails through another, or you might want a fallback transporter for reliability.

Why Multiple Transporters?

Common scenarios:

• Separate transactional and marketing emails - different providers optimize for different use cases
• Failover - if your primary SMTP goes down, switch to a backup
• Regional compliance - send emails from specific regions based on user location
• Cost optimization - use a cheaper provider for bulk sends

Configuration

Define multiple transporters in your module setup:

MailerModule.forRoot({
  defaults: {
    from: '"App" <noreply@example.com>',
  },
  // Default transporter
  transport: {
    host: 'smtp.primary.com',
    port: 587,
    auth: {
      user: 'primary@example.com',
      pass: 'password',
    },
  },
});

Then add additional transporters at runtime:

@Injectable()
export class EmailService {
  constructor(private readonly mailerService: MailerService) {
    // Register additional transporters
    this.mailerService.addTransporter('marketing', {
      host: 'smtp.marketing.com',
      port: 587,
      auth: {
        user: 'marketing@example.com',
        pass: 'password',
      },
    });

    this.mailerService.addTransporter('backup', {
      host: 'smtp.backup.com',
      port: 587,
      auth: {
        user: 'backup@example.com',
        pass: 'password',
      },
    });
  }
}

Sending with a Specific Transporter

Use the transporterName option in sendMail():

// Uses the default transporter
await this.mailerService.sendMail({
  to: 'user@example.com',
  subject: 'Order Confirmation',
  template: 'order-confirmation',
  context: { orderId: '12345' },
});

// Uses the marketing transporter
await this.mailerService.sendMail({
  transporterName: 'marketing',
  to: 'user@example.com',
  subject: 'Weekly Newsletter',
  template: 'newsletter',
  context: { edition: 42 },
});

Simple Failover Pattern

async sendWithFallback(mailOptions: ISendMailOptions) {
  try {
    return await this.mailerService.sendMail(mailOptions);
  } catch (error) {
    console.warn('Primary transporter failed, trying backup:', error.message);
    return await this.mailerService.sendMail({
      ...mailOptions,
      transporterName: 'backup',
    });
  }
}

Tips

• Use environment variables for all SMTP credentials. Never hardcode them.
• Monitor delivery rates per transporter to catch issues early.
• Set timeouts on your transporters to avoid hanging requests.

Check the Configuration docs for more details.