The Postfix configuration file in Mac OS X 10.5.5 causes Postfix to be network-accessible when mail is sent from a local command-line tool, which allows remote attackers to send mail to local Mac OS X users.
https://exchange.xforce.ibmcloud.com/vulnerabilities/45876
http://www.vupen.com/english/advisories/2008/2780
http://www.securityfocus.com/bid/31721
http://www.securityfocus.com/bid/31681
http://support.apple.com/kb/HT3216
http://secunia.com/advisories/32222
http://lists.apple.com/archives/security-announce/2008/Oct/msg00001.html