Reacher is an open-source email verification system written in Rust. It checks if an email address exists without sending any email by performing syntax validation, MX record lookup, SMTP connection with intelligent response parsing, and miscellaneous checks (disposable, role-based, gravatar, haveibeenpwned). The system is structured as a Rust workspace with four crates: a core library (`check-if-email-exists`), a CLI binary, an HTTP backend server (`reacher_backend`), and an AWS SQS Lambda worker. The backend provides REST API endpoints for single and bulk email verification, supports queue-based scaling via RabbitMQ, and stores results in PostgreSQL. It features a flexible verification method configuration allowing per-provider overrides (Gmail, Hotmail, Yahoo, etc.) with SMTP, headless browser, or API methods, and supports SOCKS5 proxies with routing rules. The codebase demonstrates strong Rust craftsmanship with clean abstractions, builder patterns, comprehensive error handling, and thorough testing.
chasquid is a production-grade SMTP (email) server written in Go, focusing on simplicity, security, and ease of operation. It handles incoming and outgoing email with full support for modern email security standards including STARTTLS, DKIM signing and verification, SPF checking, MTA-STS enforcement, and per-domain security level tracking. The server architecture is modular: the main daemon delegates SMTP protocol handling to the internal/smtpsrv package, manages a persistent email queue via internal/queue, performs alias resolution with hooks via internal/aliases, and delivers mail either locally through an LMTP-based MDA (cmd/mda-lmtp) or remotely via SMTP courier with TLS and MX fallback. Authentication integrates with Dovecot or a local user database using scrypt password hashing. Configuration is defined via protobuf text format, supporting hot reload of domains, users, and aliases without restart. The repository includes CLI tools for administration (chasquid-util), SMTP connectivity checking (smtp-check), and Dovecot authentication testing (dovecot-auth-cli). A comprehensive test suite covers unit, integration, and fuzz tests, with test data for DKIM, SPF, aliases, and various protocol edge cases. The repository also provides a Dockerfile for containerized deployment and extensive documentation in Markdown and man pages. While primarily an application, the codebase contains several well-designed internal packages that could be extracted for reuse, such as the network tracing library, safe I/O utilities, and OpenMetrics exporter.
Kurrier is a self-hosted unified communication platform designed to aggregate email, calendars, contacts, and cloud storage into a single workspace. It functions as a multi-tenant SaaS application, supporting various protocols including IMAP/SMTP for email, CalDAV/CardDAV for scheduling and contacts, and WebDAV/S3 for file storage. The system emphasizes data ownership and privacy, allowing users to manage multiple email identities and storage volumes within isolated workspaces. The architecture follows a monorepo structure with a clear separation of concerns. The frontend (`apps/web`) is built on Next.js 16 with React 19, utilizing Server Actions for business logic and a Nitro-based worker (`apps/worker`) for background jobs like IMAP sync and email delivery. Data persistence is handled by PostgreSQL with Drizzle ORM, while Redis manages job queues and caching. The platform integrates with external providers via a strategy pattern, supporting services like Google Workspace, AWS SES, and various SMTP providers. Key components include a robust provider abstraction layer for pluggable email and storage backends, a centralized encryption vault for sensitive credentials, and a standardized Server Action wrapper for consistent error handling. The codebase demonstrates high craftsmanship in its sync logic and multi-tenancy implementation, though it carries some technical debt related to type safety and complex DAV/IMAP state management.
SmsForwarder is an Android application written in Kotlin that monitors SMS messages, incoming calls, and app notifications on a device and forwards them to multiple third-party channels such as DingTalk, WeChat Work, Feishu, Telegram, email, Bark, Webhook, and others. It also provides a local HTTP server for remote control (query/send SMS, call logs, contacts, battery status, location, and Wake-on-LAN) and an automation engine with configurable conditions (cron, battery, network, Bluetooth, location, screen state, etc.) and actions (forward, send SMS, trigger alarm, control frpc tunnels, etc.). The architecture follows a standard Android pattern with Room database, WorkManager for background tasks, XUI for UI components, and a fragment-based navigation system. The codebase is large and feature-rich, with over 15 sender channels and a flexible rule engine, but exhibits some technical debt such as deprecated API usage, hardcoded vendor-specific intents, and a few large monolithic fragments.
fastapi-mail is a lightweight async Python library for sending emails from FastAPI/Starlette applications. It provides a clean, Pydantic-driven configuration model (ConnectionConfig), a message builder (MessageSchema) supporting HTML, plain text, Jinja2 templates, file attachments, CC/BCC/reply-to, and custom SMTP headers. The core FastMail class handles async SMTP delivery via aiosmtplib, with optional Redis-backed email validation utilities (DefaultChecker) and a WhoIsXmlApi wrapper. The architecture is simple and single-purpose: a library to be imported into FastAPI projects. It has comprehensive test coverage (22+ async tests) and modern async patterns throughout. The codebase is well-maintained with CI/CD, linting, and documentation. While the library does not exhibit groundbreaking innovation, it is a solid, production-ready solution for email sending in async Python web applications.
hMailServer is a production-grade, open-source email server for Microsoft Windows, implementing SMTP, POP3, and IMAP protocols with comprehensive anti-spam (SPF, DKIM, DMARC, DNSBL, SURBL, greylisting, SpamAssassin) and anti-virus (ClamAV, ClamWin, custom) capabilities. The server core is written in C++ using Boost.Asio for asynchronous I/O and exposes a COM automation API for administration. It is complemented by a .NET WinForms Administrator GUI, a PHP-based web admin, and a suite of database upgrade scripts supporting MySQL, MSSQL, PostgreSQL, and SQL Server Compact Edition. The architecture follows a layered pattern: business objects (e.g., Account, Domain, Rule) with a generic Collection/Persistence layer, a caching subsystem with TTL-based eviction, and a plugin-style spam test runner. Key data flows include message reception via SMTP, anti-spam scanning, local or external delivery, and optional virus scanning. External dependencies include OpenSSL, Boost, and database client libraries. The codebase is well-structured with reusable patterns but shows signs of age (manual memory management, COM, .NET Framework 4.5, PHP without modern framework).
This is a lightweight SMTP server written in Go, designed to receive large volumes of email, parse MIME content, and store messages in MongoDB. It includes a web interface for viewing messages, managing greylisting, and real-time notifications via WebSockets (using the embedded Incus library). The server implements core SMTP commands (HELO/EHLO, MAIL, RCPT, DATA, AUTH, STARTTLS, XCLIENT) and supports TLS, greylisting, and spam filtering via regex. The architecture is monolithic: a single binary runs both the SMTP listener and an HTTP server for the web UI. The codebase is functional but shows signs of age: it uses deprecated dependencies (gopkg.in/mgo.v2, github.com/robfig/config, github.com/gosexy/redis), lacks a go.mod file, and has no automated tests. The SMTP protocol handling is reasonably robust, and the MIME parsing is non-trivial, but the overall project would benefit from modernization and testing.