It can't be just the sender deciding on this: the receiver can require evidence too.

I just thought (@icopress) of another use case: Say you're running Awards, and you want to keep entries confidential until a certain date. You could require all participants to PM you, with a CC to "LoyceVs PM publisher" and delay=<days left>. That way the campaign manager can already see the entries, and once the contest ends, anyone can verify all entries.