Instead of deleting the message outright, try dragging the message to your deleted items folder and dropping it there and see if that removes it.
I have done all the above. The exact email keeps returning, with the same date and time. I have tried dragging it to another folder. The copy of the email goes into the folder, but shortly thereafter the same email reappears in my in box as unread.
It is not linked to any other account. Normally I access my email using IMAP on Outlook, but when the email kept reappearing, I went in and deleted it using webmail, with the same results.
