Outlook Interop fails after upgrade to Windows 10

I have a .net client program which sends emails using Interop for Outlook. This works fine on thousands of PCs on all versions of Windows. But following upgrades to Windows 10 a very small number of clients fail on sending an email with the message 'sendSystem.Runtime.InteropServices.COMException (0x80080005). Retrieving the COM class factory for component with CLSID failed etc.'. nSomething has obviously happened with the upgrade to Windows 10. The Interop interface to Word continues to work normally. I suspect a permissions issue but cannot find any guidance on this. Any comments are very welcome.