Maybe you could run a script per group policy which changes their “Display Name”
Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Profiles\PROFILENAME\9375CFF0413111d3B88A00104B2A6676\00000002
“Display Name” is a string.
you could read the proper name from active directory, which should be in:
Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity\ADUserDisplayName