Although, if you make a LOCAL folder on your mail app, and move an email into there, it will be deleted from the IMAP server and not cached anywhere, getting removed off other synced computers as well. So be careful what you move into a "local mail folder", when you do that it exists only on that computer.
I have used "on my Mac" folders for years but how do you make a "local mail folder" on an iOS device like the OP is using?
