Thanks for the reply but that's not really what I want. I don't want to delete Sent emails by default which is what (I think) the preference will do if you select "Save Sent messages to Trash". I want to probably keep some of them in the Archive folder. Even when I move them to a different folder (doesn't matter which folder.) they are moved to that folder but then they also reappear in the "Sent" folder. So then you get 2 copies of the email. One in Archive and the one that remains in the Sent folder.
I have tried selecting "Save Sent messages to Trash" and I am able to delete them and they don't return to Trash but they reappear in Sent. I only have the problem on iCloud. Any Sent email from my iPhone acts normally.
Is it possible something is wrong with the Sent folder itself and maybe needs a rebuild or repair which I have no idea how to do.
Thanks, Jack