Q: Can't Delete A Contact In Mail 6.6

In Mail 6.6 I cannot delete a contact, although it is not in my Contacts app. When I try to enter the contact's email while composing the email, I get the correct email address + the email address of the, in this case, what I suspect is a Malware email address which I am trying to delete permanently. I have imported the contents of my Contacts book into my Gmail account + my ISP webmail account, the problem does not occur, I only get the legit email of the contact I am writing to. How can I permanently delete this Malware email listing? Please make your reply easy to follow. Thanks, Ken.