Postfix 1.1.11 and earlier allows remote attackers to use Postfix to conduct "bounce scans" or DDos attacks of other hosts via an email address to the local host containing the target IP address and service name followed by a "!" string, which causes Postfix to attempt to use SMTP to communicate with the target on the associated port.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A522
http://www.securityfocus.com/bid/8333
http://www.redhat.com/support/errata/RHSA-2003-251.html
http://www.novell.com/linux/security/advisories/2003_033_postfix.html
http://www.mandriva.com/security/advisories?name=MDKSA-2003:081
http://www.debian.org/security/2003/dsa-363
http://secunia.com/advisories/9433
http://marc.info/?l=bugtraq&m=106001525130257&w=2
http://distro.conectiva.com.br/atualizacoes/?id=a&anuncio=000717