Post Hijacking
From JumbaWiki
Post Hijacking is a relatively new avenue for spammers to target poorly scripted form pages allowing them to post spam from innocent websites, hence blacklisting the website details and remaining anonymous.
For more information about the security exploit and how to write your form pages securely to avoid it, check out http://www.anders.com/projects/sysadmin/formPostHijacking/ . This is also a good article on e-mail header injection: http://www.nyphp.org/phundamentals/email_header_injection.php
