What the upgrade involved

The supported-OS floor moved to Windows 10 1607, the .NET 8 runtime's own minimum, and the obsolete .NET Framework 4.5 gate is gone. The .NET 8 Desktop Runtime became a server-component prerequisite, not just a Control Panel one, and its exit code is checked before the database tools run at post-install.

Changes

• DBSetup, DBSetupQuick, DBUpdater, DataDirectorySynchronizer and the Shared library moved from .NET Framework 4.8.1 to SDK-style net8.0-windows. The COM API is consumed through a checked-in tlbimp wrapper in source/Tools/Interop/, so the tools build with plain dotnet build and no registered typelib. The silent command-line contract the installer depends on is unchanged.
• The five VB6 migration wizards are gone. None could be built, their shared VB6 sources were never in the repository, and three migrated from products dead for two decades. The two worth keeping return as one .NET 8 Import Tool under Addons. Accounts from a comma-separated text file get per-line validation and a created/updated/failed summary instead of a crash on the first malformed line. Messages from mbox files stream through the supported COM API, Utilities.ImportMessageFromFileToIMAPFolder, one IMAP folder per file, rather than raw MySQL INSERTs. The silently dropped last message, CRLF mailboxes parsing as one giant message, mboxrd >From quoting and SMTP dot-stuffing corrupting stored messages are all fixed.
• CI now builds every C# project, with warnings-as-errors on the tools solution. Previously only the Control Panel was built, which let the NUnit 4 bump break the test and tool projects invisibly.
• The test suite runs on NUnit 4.6.1, with the 2,200+ classic assert call sites kept compiling through NUnit 4.6's C# 14 extension members. Local-address selection now probes for an address the server answers on instead of trusting interface enumeration order, which broke under a connected VPN.

The full regression suite passed 1026 of 1026 against the rebuilt service, with live SpamAssassin and ClamAV.

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