There is a wonderful feature in Spark and Bluemail that allows the user to share an email with a pre-approved contact in lieu of forwarding and going back and forth that way. When you share an email with another user, a chat opens up in which the two users can discuss the content of the email without having to forward and comment in a new email. The absence of this feature is the ONLY reason my wife and I haven’t switched to protonmail desktop as our primary email client.