My suggested CCleaner feature is to clean the autocomplete addresses within Outlook. That is, where you begin to type an email address in a new message and Outlook suggests previous addresses you have mailed, by virtue of having stored these previous addresses. This is even where those addresses are not deliberately stored in the address book.
I don't know where in the registry or wherever these addresses are stored, but from what I can tell from reading through some of the forums, there are plenty of people who will know or can find this out.