Mailsender – Scalable SMTP Email Service with Template Engine

January 18, 2026

Delivery snapshot

Record date: January 18, 2026. Technology and screenshots describe the delivered version documented on this date. Any performance observations belong to the same point-in-time record. The linked live site may include later changes made by the client or other providers.

Overview

Mailsender is a production-grade transactional email service built on Node.js, Express, and Nodemailer. It replaces per-project email configurations across agency work with a centralized, scalable platform featuring connection pooling, template rendering, bulk sending, and a full React admin UI. The service supports multiple SMTP providers through a configurable transporter map, with pure being the default. Retry logic with exponential backoff handles transient network failures gracefully, while connection pooling maximizes throughput for high-volume sending. A React-based frontend (using Radix UI, Lucide icons, and Tailwind CSS) provides project management, recipient list handling, and real-time metrics.

Key Features

Multi-Provider SMTP Support

The emailService.js module maintains a Map of named transporters. Adding a new provider is a config change, not a code change. Each transporter is independently configurable with its own host, port, credentials, and pool settings.

Connection Pooling

Nodemailer's built-in pool manages concurrent connections, max messages per connection, and rate limiting. The configuration (pool, maxConnections, maxMessages, rateDelta, rateLimit) is tunable per provider without code changes.

Handlebars Template Engine

Templates live in /templates as .hbs files with layout support (base.hbs), partials, and custom helpers (formatDate, uppercase, eq, json). Variable substitution works for both single sends and bulk batches.

Bulk Sending with Batching

The POST /api/v1/email/send-bulk endpoint accepts an array of recipients, each with their own data payload. A configurable batchSize controls how many emails are dispatched per connection cycle, preventing SMTP throttling.

Retry with Exponential Backoff

Transient errors (ECONNREFUSED, ETIMEDOUT, ENOTFOUND) trigger up to three retry attempts with exponential backoff (2s → 4s → 8s). The sendEmailWithRetry() function wraps all sends and logs retry attempts to the metrics service.

Full Auth Layer

JWT-protected routes with jsonwebtoken and bcryptjs for password hashing. Session cookies store the JWT for browser-based access, while API keys handle programmatic requests.

Metrics Tracking

The metricsService.js tracks retry counts, delivery success/failure rates, and per-project volumes. Metrics are queryable via GET /api/v1/metrics for dashboard visualization.

Dev Mode

Setting DEV_MODE=true in development logs all emails to the console instead of sending them. This lets us test templates and flows without touching real inboxes.

Technologies Used

Express v4 – HTTP server and routing
Nodemailer v6 – SMTP client with pooling
Handlebars – Template engine with layouts and partials
React – Admin UI frontend
Radix UI – Accessible component primitives
Tailwind CSS – Utility-first styling
Lucide React – Icon library
JWT + bcryptjs – Authentication
Multer – File uploads (recipient lists)
Sharp – Image processing for inline attachments
Helmet + CORS + rate-limit – Security middleware

Challenges & Solutions

SMTP provider reliability
Different providers have different limits and behaviors. Abstracted the transporter into a configurable map so we can switch providers or add fallbacks without rewriting the sending logic. Bulk send performance
Sending hundreds of emails in a single request overwhelms SMTP connections. Implemented configurable batching to stay within provider rate limits while maximizing throughput. Template flexibility
Different email types need different layouts. Built a Handlebars layout system with partials and custom helpers so each template inherits a consistent base while supporting unique content.

Impact

Centralized email infrastructure across 10+ client projects
Scalable throughput via connection pooling and batching
Reliable delivery with automatic retry on transient failures
Template consistency with a shared Handlebars foundation
Full observability via metrics and delivery logs