Many websites that offer internal (non-internet) mail, show if the reciepient have opened/read the mail and/or replied to it. Typical examples are Myspace (see screenshot), some dating websites, etc.
Sometimes in a Notes environment you send a mail, and then you realize a few hours later, when you don't hear anything back, that you should have added delivery confirmation or conformation that the other person opened the mail. But those functions also add additional e-mail to my mailbox...
Why not simply have a flag in the sent folder (and in the Notes 8.x mail threads) that show:
* if the mail has been opened (read)
* if the mail have been responded to (replied)
* if the mail have been sent but not acted on by the user (sent)
This would cut down on the number of extra email, and make the inbox easier to maintain, and at the same time, give the user better control of sent mail.