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