Mail in Apple OS X before 10.10 does not properly recognize the removal of a recipient address from a message, which makes it easier for remote attackers to obtain sensitive information in opportunistic circumstances by reading a message intended exclusively for other recipients.
https://support.apple.com/kb/HT6535
https://exchange.xforce.ibmcloud.com/vulnerabilities/97629
http://www.securitytracker.com/id/1031063
http://www.securityfocus.com/bid/70619
http://archives.neohapsis.com/archives/bugtraq/2014-10/0101.html