The solution posted did not work for me as there was no "account" Account option after I clicked on "Accounts & Password". I am assuming they were changed in an update.
After messing with this for over an hour, I discovered that when I clicked an option (I apologize, I can't remember which one exactly- might have been the curving arrow for forwarding or printing) on one of the offending emails, there was a "see other options" or "see related items" link that isn't normally there, and didn't show on other emails. When I clicked that link it gave me an "available threads" screen and the email was listed as a "thread". I have no idea how or why. However, once I selected it there and clicked the trashcan icon, it deleted and allowed me to delete the other few random emails that previously kept coming back. I hope this helps.