That is a feature of the old Zimbra email program. It appears in the auto-fill when you are typing into the To: field of a new email you are composing----right? It is based on the frequency of usage of the most common recipients you send emails to. It will rank them based on that usage. What it does is place those addresses in the top 6 or 8 slots in the list when you start typing. If you click on "Forget", all it does is tell the auto-fill to disregard the rank and send that address further down in the list. At any rate, you will at some point be migrated to the new Appsuite format, and that does not have that feature.