6.0.0 is the first modernised release of the fork: 64-bit only, built with the VS2026 toolchain, with OpenSSL 4 and TLS 1.2 and 1.3 underneath. Build 4 is the production build, and it fixes the installer defect that made build 3 unusable on fresh installations.
What the upgrade involved
6.0 was 64-bit only. The installer dropped the obsolete 2015-era UCRT, which had been included in Windows since Server 2016 / Windows 10. Upgrade notes and the full feature list are in the README.
The installer fix
Build 3 bundled VS2019-era VC++ runtime DLLs. The service loaded those instead of the current ones and crashed at startup with an access violation in msvcp140.dll on fresh installations. Build 4 ships the matching v145 (14.51) runtimes. Build 3 also fixed MySQL/MariaDB database creation, which was a script line-ending problem, and bundles libmysql.dll.
What 6.0 brought
OpenSSL 4, TLS 1.2 and 1.3
MTA-STS and DANE, with in-process DNSSEC validation
ARC sealing and Ed25519 DKIM
ACME (Let's Encrypt) built in
REST admin API and Prometheus metrics
IMAP MOVE, ID and SPECIAL-USE
PBKDF2 password hashing
The release was validated by the full regression suite: 898 of 898 tests passing, zero failures, run against a MariaDB (MySQL backend) instance, including live SpamAssassin, ClamAV with real EICAR detection, DMARC against live DNS, and TLS 1.2 and 1.3 end to end.
Full release notes, checksums and signatures
