Using Multiple SMTP Transporters in Production

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.