"Seen" notification that differentiates between all recipients

The “Seen” notification in the bottom corner of emails and messages will change to the “Seen” status even if only one person in the thread has viewed the email. It would be great to be able to see which recipients have opened the message. Sometimes my colleagues are included on email threads, but I need to know when the client is the one who opened the email. This would really help with follow up messages, to know if the client has read my email yet or not. It could say “Seen by [first name]” or “Seen by [colleague] and [client]”