Outlook Express 6.0, when sending multipart e-mail messages using the "Break apart messages larger than" setting, leaks the BCC recipients of the message to the addresses listed in the To and CC fields, which may allow remote attackers to obtain sensitive information.
https://exchange.xforce.ibmcloud.com/vulnerabilities/17098
http://www.securityfocus.com/bid/11040
http://www.networksecurity.fi/advisories/outlook-bcc.html
http://support.microsoft.com/kb/843555