Hi G, thanks for taking the time to create a new thread I really appreciate it. I'll get this raised for you now. OX aren't the fastest when the ball is in their court, but I'll do my best for you.
A minor update from me.
Draft mail messages not being deleted on sending remains unresolved for TalkTalk webmail and the TalkTalk mobile app (android).
However, when Thunderbird (IMAP client) saves a Draft then that saved draft appears in the online mailbox Drafts folder. When Thunderbird sends the Draft the saved draft in the online mailbox Drafts folder is marked for deletion because the subject and recipient text is crossed out in the List view. When sent the message is copied to the Sent objects folder and the saved Draft is ultimately deleted.
So, when working with an email client the Drafts are deleted on sending which is the normal expected action.