This feature caches, in memory, a list of recipient names that are frequently used. When you begin typing a name into any address field that expects an e-mail address, the cached entries that match the letters you have typed are displayed below the address field in the type-ahead dialog box.
If you want to search a directory for a name, you can click either the check name icon or the "Search Directory for" text at the bottom on the type ahead dialog box.
To turn off the type-ahead feature or to remove any cached list entries, click Preferences and then expand Basics and click Type-ahead. If your administrator has disabled the type-ahead feature, the type-ahead panel will be grayed out and all actions will be disabled.