You said should use a new email address, and make that my Apple ID, yet in your second answer, you say "you can never change your Apple email address", so..... change it, or I can't change it? And what do you mean by using it for Apple & only Apple? I'm not sure how using a different email address only for Apple, will solve the issue of other people using my iCloud mail email addresses.
I looked in my Sent folder, and only messages I've sent are there, so that doesn't tell me anything. Well, except make it more confusing, cause obviously, if others are sending emails using my address, they should show up in the Sent folder, just as when others respond to them, those messages end up in my inbox! And if they end up in mine, how & why are they also ending up in the other people's inboxes, if they don't have the addresses I got via Apple? It's totally bizarre, I get responses to their actions, by the parties they've interacted with, yet the messages they send aren't in my Sent folder, and why do we both receive responses?