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.