Apparently, spammers are now exploiting a hole, or holes, in multiple PHP scripts which use the mail() API.
The holes are described at the SecurePHP wiki; basically, the script author inserts CGI fields directly into a message template without stripping newlines, and this allows attackers to create new headers, take over the message body, and generally take over the mail message and destinations entirely.
Funnily enough, these are the same holes Ronald F. Guilmette and I found in FormMail 1.9, and described in our Jan 2002 advisory Anonymous Mail Forwarding Vulnerabilities in FormMail 1.9 (PDF) on page 10, Exploitation of email and realname CGI Parameters. Ah, plus ca change…
Worth noting that perl’s venerable taint checking would have spotted these, if it were used.