Hi Stefan,
Engineering did get back to me and they will implement a fix to check whether the default client supports Extended MAPI and if not, use SImple MAPI. I've opened CR 781930 to add this check into the MAPI implementation.
As these are not email clients we use, could you please export the registry entries for these mail clients from this registry key below.
HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail
Please rename the .reg files to .txt and attach them here.
Thanks,
Pat