Durning testing, i checked out how far down a hMailServer 64-Bit Installation will work with the latest production codebase (from 6/23/2018)
and OpenSSL 1.0.2.o.
Result: It runs on Windows XP SP2 64-Bit
Notes:
*The integrated .NET Installer in the hMailServer Installer cannot install the .NET 2.0 Framework by itself and the Installation will fail
*The workarround is, to download .NET 3.5 Standalone Installer for Windows XP 64-Bit manually and install it
*MS-SQL CE 3.5 x64 on Windows XP SP2 64-Bit works, if you install it manually first
Screenshot: