When sending mail, Outlook adds the mail address to a temporary list of users, when resending, it allows an "Auto Complete" function when the first few letters of a mail address have been typed.
To back this up you will need to show your hidden files and locate the file here...
Documents and Settings / %user% / Application Data / Microsoft / Outlook / .nk2
in windows 7
Users / %user% / Application Data / Microsoft / Outlook / .nk2