Postfix 2.1.3, when /proc/net/if_inet6 is not available and permit_mx_backup is enabled in smtpd_recipient_restrictions, allows remote attackers to bypass e-mail restrictions and perform mail relaying by sending mail to an IPv6 hostname.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A11339
https://exchange.xforce.ibmcloud.com/vulnerabilities/19218
http://www.securityfocus.com/bid/12445
http://www.redhat.com/support/errata/RHSA-2005-152.html
http://secunia.com/advisories/14137/