Down arrow and delete. That's the official way to delete them from autocomplete memory. That's independent of your contacts file.
I have a lot of contacts in my Microsoft Outlook. I also have a lot of people who have changed emails, or their emails arent checked anymore. I changed these peoples emails in my contacts list, but the unused emails still come up in autocomplete. Usually as the first choice too. Is there a way to make the autocomplete only consult my contacts, or anything like that?