message.rpmsg not able to open in outlook 365 or Outlook Desktop app?

I have a user who has got an encrypted message message.rpmsg addresssed to a business address in in O365 tenant (no on-prem stuff)

She cannot open it : not in Office365 nor in Outlook 365 Desktop

(I think it is normal she can't open it on a personal iPhone).

But I am a but suprised that Outlook ow OWA in the tenant will not open it. 

Maybe it it an old version - I see the recent versions are message_v4.rpmsg