Mail in Apple Mac OS X before 10.9, when Kerberos authentication is enabled and TLS is disabled, sends invalid cleartext data, which allows remote attackers to obtain sensitive information by sniffing the network.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2013-5023
http://lists.apple.com/archives/security-announce/2013/Oct/msg00004.html