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