Outlook suggests previously entered addresses in the "To", "Cc" and "Bcc" fields when entering a new address.
Often, these addresses are no longer relevant and serve to annoy rather than make life easier. To delete individual addresses, select the entry and press the Del key.
To switch off this function completely go to Tools > Options > E-mail Options > Advanced E-mail Options and uncheck "Suggest names while completing To, Cc, and Bcc fields".
