MimeKit is a mature, high-performance .NET library for creating, parsing, and manipulating MIME messages (email). It provides comprehensive support for all standard MIME structures (multipart, message/rfc822, content-transfer encodings), internationalized headers (RFC 2047/2231), and email security features including S/MIME, OpenPGP, DKIM, and ARC signing/verification. The library also includes a full HTML5 tokenizer, TNEF (Outlook Transport Neutral Encapsulation Format) reader, text format converters (HTML, flowed, plain), and a stream filter pipeline for encoding/decoding and line-ending normalization. The codebase is well-architected with clean abstractions (visitor pattern, filter chain, encoder/decoder interfaces), extensive unit tests, and benchmarks demonstrating 25x–75x faster parsing than competing .NET MIME libraries. It is distributed as NuGet packages (MimeKit, MimeKitLite) and is a .NET Foundation project. The repository also includes sample applications for Windows Forms, iOS, and Android, and a DKIM verifier sample.
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.
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).