Stalwart v1.0: The Fastest Mail Server on the Planet

Oct 26, 2026 - 16 min read Five years ago, on 3 October 2021, the first commit landed in a small repository called mail-parser. It was an e-mail parsing library written in Rust, and nothing more. Over the following five years it grew into a complete mail and collaboration server with JMAP, IMAP, POP3, SMTP, CalDAV, CardDAV and WebDAV, a spam filter, clustering, and support for half a dozen storage backends. Today, after five years of continuous development, we are proud to announce Stalwart v1.0, the first stable release. Version 1.0 was never meant to be another feature release. It had two goals. The first was performance: making every part of the server as fast as it can be, and proving it with numbers. The second was stability of the data itself: finalising the database schema so that, from now on, upgrading Stalwart means installing a new version and nothing more, without the disruptive migrations some earlier releases required. Along the way a few new features found their way in too, including full-text search inside PDF and office documents. Let’s walk through it. Months spent chasing every microsecondSection titled “Months spent chasing every microsecond” Most of the work in this release happened under a profiler. For months we measured where Stalwart spends its CPU cycles, its memory and its database round trips, rewrote whatever stood out, and measured again. The release contains close to 200 commits touching more than 1,300 files, and the vast majority of them exist to make something faster or lighter. Parsers rewritten with SIMD and SWARSection titled “Parsers rewritten with SIMD and SWAR” A mail server spends a surprising share of its time looking for specific bytes: the end of a header line, a MIME boundary, an equals sign in a quoted-printable body, the space that ends an IMAP tag. Done one byte at a time, that is a lot of loop iterations for a 10 MB message. Two techniques let a CPU do this work many bytes at a time: SIMD (Single Instruction, Multiple Data) uses the wide vector registers of modern processors (SSE2, SSSE3 and AVX2 on x86-64, NEON on ARM) to compare or transform 16 or 32 bytes with a single instruction. SWAR (SIMD Within A Register) applies the same idea with ordinary 64-bit integers: a few carefully chosen arithmetic and bitwise operations test all eight bytes of a word at once. It needs no special instructions, so it works on every CPU. Stalwart now uses both, everywhere it matters. At the bottom of the stack sit two …