The following process should be done to ensure that your iPhone uses the folders on the server to store Drafts, Sent and Deleted emails as by default it stores these on the phone itself.
Settings -> Accounts & Passwords -> (NetHosted Email Account) -> (Account Name) -> Advanced -> Sent Mailbox -> On the Server, scroll down inside the Inbox folder choose the Sent folder
This process just needs the last step adapted for the Drafts and Deleted folders.