Postal is an open-source mail server designed for developers and DevOps teams who need full control over their email infrastructure. It replaces proprietary services like SendGrid, Mailgun, or Postmark by providing a complete SMTP solution for sending, receiving, and monitoring email at scale. Built for web applications, it eliminates third-party dependencies and ensures data privacy.
Built in Ruby and Docker-based, Postal supports SMTP, IMAP, and API-driven email delivery with real-time analytics and webhooks. It can be deployed on any Linux server using provided installation scripts or Docker Compose, and integrates with existing infrastructure via REST APIs and webhooks for event tracking.
What You Get
- SMTP & IMAP Server - Full SMTP and IMAP protocol support for sending and receiving email with custom domain configuration.
- Email API - RESTful API to programmatically send emails, manage queues, and retrieve delivery status without relying on third-party services.
- Real-time Analytics Dashboard - Visualize email delivery rates, bounces, opens, and clicks with detailed per-message metrics.
- Webhook Integration - Receive real-time event notifications (delivered, opened, clicked, bounced) via HTTP POST to your endpoints.
- Multi-Tenant Support - Manage multiple domains and users with isolated configurations and permissions within a single instance.
- Docker & Scripted Installation - One-line install scripts for Ubuntu and Docker Compose for rapid deployment in dev or production environments.
Common Use Cases
- Running a SaaS application with transactional emails - A startup uses Postal to send password resets, confirmations, and notifications without paying per-email fees to SendGrid.
- Email marketing with full data ownership - A digital agency hosts its own email campaigns using Postal to avoid data leaks and comply with GDPR.
- Monitoring email deliverability for enterprise systems - A financial institution deploys Postal to track bounce rates and spam scores for compliance-critical outbound emails.
- Developing email infrastructure for microservices - A DevOps team uses Postal’s API to integrate email delivery into their containerized service stack without external dependencies.
Under The Hood
Architecture
- Monolithic Rails application with MVC structure where controllers tightly couple HTTP routing, authentication, and business logic, violating separation of concerns
- ActiveRecord models embed domain logic, DNS resolution, and side effects via callbacks, blurring data access and business rules
- Absence of dependency injection or repository patterns leads to tight coupling with global configuration and direct database calls
- Legacy API layers and frontend assets are preserved without clear abstraction boundaries, relying on Rails asset pipeline with minimal encapsulation
- Authentication and authorization logic is duplicated across controllers rather than centralized in a dedicated service or policy layer
Tech Stack
- Ruby and Rails form the core backend, leveraging ActiveRecord and the asset pipeline for frontend asset management
- MariaDB serves as the primary database, configured via Docker with environment-driven connectivity
- Puma powers the production server, integrated into a Dockerized deployment pipeline with a custom entrypoint
- Comprehensive testing tools including RSpec, FactoryBot, and WebMock ensure robust validation of email delivery behaviors
- Asset compilation is handled through Rails conventions with Uglifier and Autoprefixer, precompiled during Docker builds
- CI/CD automation uses semantic versioning and style enforcement via RuboCop to maintain code consistency
Code Quality
- Extensive test coverage spans unit, integration, and edge cases with clear component-based organization
- Factory-based test data generation ensures consistent, readable test scenarios including complex states like suspended organizations
- Domain-focused naming and logical separation between parsing, validation, and delivery components enhance readability
- SMTP protocol behaviors are thoroughly tested, including edge cases like message size limits and loop detection
- Reliance on Rails’ default error handling and schema-defined models reduces explicit exception handling but maintains predictability
What Makes It Unique
- Customizable IP prioritization and weighted email distribution enable sophisticated warm-up and load balancing without external dependencies
- Dynamic domain and tracking domain management with built-in DNS validation provides granular control over email deliverability
- Unified IP pool architecture allows multi-IP management with fine-grained priority rules, a rare capability in open-source email platforms
- Admin interface features theme-aware, configurable components that maintain visual consistency while supporting complex multi-tenant workflows
- Server-level configuration with conditional UI elements creates a tailored, role-aware administrative experience for email infrastructure management