Wietse Venema : Security Vulnerabilities, CVEs, Published In 2003

The address parser code in Postfix 1.1.12 and earlier allows remote attackers to cause a denial of service (lock) via (1) a malformed envelope address to a local host that would generate a bounce and contains the ".!" string in the MAIL FROM or Errors-To headers, which causes nqmgr to lock up, or (2) via a valid MAIL FROM with a RCPT TO containing a ".!" string, which causes an instance of the SMTP listener to lock up.

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.