What happens when an external recipient receives a Microsoft OME email?

Proofpoint Email

The recipient will be given seven days to read the message.  If the user does not have a Microsoft account, they will be asked to register or request a one time passcode in order to gain access to the message:

 

Proofpoint Account Setup

 

Otherwise, they will be directed to sign-in to their Microsoft Account in order to view the encrypted message.