When filling in the “To:/Cc:/Bcc” fields in an e-mail message, Outlook will provide you with “Auto Complete” entries. This will be a list of e-mail addresses that are similar to what you are typing (e-mail address that you have used before). It makes it quick for you to left-click on of the “Auto Complete” entries to enter an address into the field. Some of the e-mail addresses in the “Auto Complete” entries may no longer be valid, you will want to delete those entries.