Reason number 8 is:

> Improvements to Outlook Express help reduce
> unwanted e-mail by limiting the possibility
> of your e-mail address being validated by
> potential spammers.

Except when it does the opposite. When you forward a complaint to the administrators of the spammer's hosting ISPs, Outlook Express still forwards to the spammer the same validations that it used to do when displaying the original spam.
http://support.microsoft.com/default.aspx?scid=kb;en-us;843018&product=windowsxpsp2