Outlook remembers addresses of people you have sent e-mail to, so it will remember all of the old addresses and try to autocomplete them.
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. To remove an erroneous entry, after you start to type the name:
• Select the unwanted name or e-mail address by using the UP ARROW or DOWN ARROW key.
• Press DELETE.