If I click my "To:" button to get a listing of contacts to add to a new email, I get a number of strange email addresses listed that are not in my regular "Contacts" file; how do I get rid of them?

If I click my "To:" button to get a listing of contacts to add to a new email, I get a number of strange email addresses listed that are not in my regular "Contacts" file; how do I get rid of them?  Suspect hacking, so changed to an extremely complex password, strange email addresses still display in the "To:" dropdown listing.