That would be the Email Autocomplete Cache. To disable it:

Click on File and go to Options.
Click on Mail on the left.
Scroll down to the Send Messages section.
Uncheck the box for "Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines"
Click OK.

You can also clear the cache by clicking the "Empty Auto-Complete List" button

That should be it. Hope this helps.

-SW