External users cannot reply to encrypted emails

So we are trialing Azure Information Protection as a potential solution for our HR department to be able to send out encrypted emails that contain confidential information.  In our testing, we have a label set up that applies the encryption.  In the label permissions, we have it set up to allow the external address we are testing with to view the email, and permissions are also set to allow them to reply to the email.  The problem is when you reply from the external address, in the web browser interface viewing the encrypted message, the reply goes nowhere.  Is there some addition permissions that need to be set to allow this?  The external address has VIEW,DOCEDIT,REPLY,REPLYALL,FORWARD,and OBJMODEL.