AutoComplete is a feature which displays suggestions for names and e-mail addresses as you begin to type them. These suggestions are possible matches from a list of names and e-mail addresses that you have typed before, known as the AutoComplete name list.
On the Tools menu, click Options.
On the Preferences tab, click E-mail Options, and then click Advanced E-mail Options.
Select or clear the Suggest names while completing To, Cc, and Bcc fields check box.