The Mail Drop feature in Mail in Apple OS X before 10.11 mishandles encryption parameters for attachments, which makes it easier for remote attackers to obtain sensitive information by sniffing the network during transmission of an S/MIME e-mail message with a large attachment.
https://support.apple.com/HT205267
http://www.securitytracker.com/id/1033703
http://www.securityfocus.com/bid/76908
http://lists.apple.com/archives/security-announce/2015/Sep/msg00008.html