Requil

API-first email engine for generation, validation, rendering, and delivery

Live at: requil.app

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Overview

Requil is an email platform that consolidates the entire email workflow into a single, cohesive system. It combines template creation, quality validation, cross-client rendering, and reliable delivery into one unified API-first engine.

The core pipeline:

Template Creation → Validation & Rendering → Transport Configuration → Email Delivery

Instead of juggling multiple tools for building emails, validating code, rendering templates, and managing sending infrastructure, Requil provides a single source of truth for all email operations across your applications.

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Core Concept

The Four-Stage Pipeline

1. Template Creation

   • Build emails using React Email components with Liquid templating
   • AI-assisted generation from prompts and brand guidelines
   • Drag-and-drop editor with built-in guardrails

2. Validation & Rendering

   • Hard validation: WCAG contrast, alt-text coverage, HTTPS enforcement, HTML size limits
   • Per-recipient rendering with variable substitution
   • Automatic plaintext generation
   • Immutable snapshots for predictable deployments

3. Transport Configuration

   • Unified transport layer supporting Resend (BYOK) and custom SMTP
   • SPF/DKIM setup wizard for deliverability
   • Suppression lists and bounce handling
   • Rate limiting per workspace and transport

4. Email Delivery

   • Single /v1/send endpoint with idempotency guarantees
   • Automatic retry with exponential backoff via QStash
   • Fan-out for large campaigns with batch checkpointing
   • HMAC-signed webhooks for delivery events

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Architecture

Monorepo Structure

Requil is organized as a Turborepo monorepo with clear separation between applications and shared packages:

Applications:

• apps/api - Fastify monolith (API + rendering + worker endpoints)
• apps/dashboard - Next.js App Router (admin interface)
• apps/website - Astro/Next.js SSG (marketing site, planned)

Shared Packages:

• packages/email-engine - React Email + Liquid rendering, plaintext generation, guardrails
• packages/transports - Resend SDK and SMTP adapters
• packages/validation - AJV schemas and quality rules
• packages/webhooks - HMAC signature generation and verification
• packages/ratelimit - Upstash Redis rate limiting and idempotency
• packages/db - Drizzle ORM with migrations
• packages/types - Shared TypeScript types
• packages/utils - Logger (pino), crypto, error handling
• packages/config - Shared configs (tsconfig, vitest, biome)

Backend Architecture

The API follows feature-based folder structure with CQRS (Command Query Responsibility Segregation) and DDD (Domain-Driven Design) principles:

Feature-Based Organization:

src/modules/
  ├── auth/           - Authentication & authorization
  ├── workspace/      - Multi-tenant workspace management
  ├── templates/      - Template CRUD and snapshots
  ├── api-keys/       - API key management
  └── ...

CQRS Pattern:

• Commands: Write operations (create, update, delete) with side effects
• Queries: Read operations without side effects
• Dependency injection via Awilix
• Command/Query Bus for decoupled execution

Domain-Driven Design:

• Domain entities with business logic encapsulation
• Value objects for type-safe primitives
• Repository pattern for data access
• Domain events for cross-module communication

Infrastructure:

• Database: Supabase (Postgres with RLS) + Drizzle ORM
• Cache/Queue: Upstash Redis (rate limiting, idempotency, cache) + QStash (retry, scheduling)
• Observability: pino JSON logging with traceId, Sentry (planned)

Frontend Architecture

The dashboard uses Next.js 16 App Router with feature-based organization:

Features:

• features/auth - Login, registration, OAuth
• features/editor - Drag-and-drop email builder (55+ components)
• features/templates - Template management
• features/workspace - Workspace settings

State Management:

• Zustand for client state
• RxJS for reactive patterns
• React Hook Form + Zod for forms

Styling:

• Tailwind CSS 4
• shadcn/ui (Radix UI primitives)
• Dark mode support via next-themes

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Tech Stack

Core Technologies

Backend:

• Node.js 22+, TypeScript 5.9+, Fastify 5
• Drizzle ORM, Supabase (Postgres/Auth/Storage)
• Upstash Redis, QStash

Frontend:

• Next.js 16 (App Router), React 19
• Tailwind CSS 4, shadcn/ui
• Zustand, RxJS

Email Rendering:

• React Email (TSX components)
• Liquid templating (liquidjs)
• html-to-text (plaintext generation)
• Cheerio (HTML analysis)

Transports:

• Resend SDK
• Nodemailer (SMTP)

Quality Tooling:

• BiomeJS 2.2+ (lint + format)
• Vitest (unit + integration)
• Playwright (E2E)
• Turbo 2.6+ (monorepo orchestration)

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Key Features

Implemented ✓

• Multi-tenant workspace management
• Template CRUD with React Email components
• Asset management (upload, storage)
• API key authentication with scopes
• Drag-and-drop email editor
• OAuth (Gmail, Github) + credentials authentication
• Rate limiting per workspace
• HMAC-signed webhooks
• i18n support (EN/PL)

In Development ⏳

• /v1/send endpoint with idempotency
• Template snapshot publishing (immutable versions)
• QStash retry with exponential backoff
• AI-assisted template generation
• WCAG contrast validation with auto-fix
• Suppression list management
• Usage tracking and billing

Planned (Post-MVP)

• Newsletter campaigns (double opt-in, segments)
• Advanced analytics and dashboards
• A/B testing
• Journey builder
• Multi-channel support (SMS, push)

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Project Status

🚧 Active Development - This project is in continuous development and serves dual purposes:

• Building a production-ready email platform
• Educational exploration of modern full-stack architecture patterns

Expect frequent changes, breaking updates, and ongoing experimentation with best practices in monorepo architecture, DDD, CQRS, and scalable email infrastructure.

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Contributing

This is a personal project primarily for learning and experimentation. However, feedback, suggestions, and discussions are always welcome. Feel free to open issues or reach out with ideas.

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License

MIT License

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Built as a learning journey into modern email infrastructure and full-stack architecture.