Stalwart 1.0 stable release brings SIMD-optimized Rust mail server with JMAP, CalDAV, CardDAV

Stalwart, an open-source mail server written in Rust, has released version 1.0 after five years of development, marking its transition from beta to stable. The software functions as an integrated platform handling SMTP, IMAP, POP3, and JMAP protocols while also providing calendaring, contacts, and file storage through CalDAV, CardDAV, and WebDAV.

The 1.0 release focuses heavily on performance engineering, with core parsers rewritten using SIMD and SWAR techniques for parallel byte processing. The developers report their mail-parser library outperforms Rust's mailparse by up to three times and Dovecot's parser by nine times. Database operations have been streamlined across RocksDB, PostgreSQL, MySQL, SQLite, and FoundationDB backends, with zstd compression tuned for email and PIM data. A redesigned caching layer reduces database access for common IMAP operations including sorting, threading, and flag retrieval.

Performance engineering and protocol additions in Stalwart 1.0

• SIMD/SWAR parser rewrite: Core parsers now process multiple bytes simultaneously rather than character-by-character, with the mail-parser library reportedly achieving 3x speedup over mailparse and 9x over Dovecot's parser.
• Database and storage optimizations: Reduced unnecessary storage operations, more efficient transactions, zstd compression specialized for email/calendar/contact data, and backend-specific tuning for RocksDB, PostgreSQL, MySQL, SQLite, and FoundationDB.
• Full-text search engine: Completely rewritten with phrase search, word stemming, prefix matching, and compressed indexes; benchmarked at under four milliseconds to search 250,000 messages.
• Document indexing pipeline: New capability to extract and index text from PDF, Microsoft Office, OpenDocument, EPUB, and RTF files stored as attachments or in integrated file storage.
• Cryptographic and authentication updates: Post-quantum DKIM2 signatures combining ML-DSA-44 and Ed25519, expanded OAuth/OpenID Connect support, and SSRF protection added.

Why it matters

Stalwart 1.0 offers Linux system administrators a single-binary alternative to the traditional Postfix/Dovecot stack with modern protocol support including JMAP, which few open-source servers implement natively. The project's Rust codebase and claimed performance advantages may attract deployments where resource efficiency matters, though the one-time migration requirement from 0.16.x and finalized schema represent a significant commitment point for early adopters evaluating it against battle-tested alternatives.