cannot delete mail from storage in usage section of iphone?

7 iPhone Mail Settings You Need To Change Now

Try going into Settings > Accounts & Password > account > Account > Advanced > Deleted Mailbox, and under On The Server tap the ‘Trash’ folder so that it gets a tick against it – then come out of that series of popups by tapping on the relevant buttons at the top of them and see if you can now delete emails.

Messages in iCloud – Storage and How It Works