The smtp_filter function in spamdyke before 3.1.8 does not filter RCPT commands after encountering the first DATA command, which allows remote attackers to use the server as an open mail relay by sending RCPT commands with invalid recipients, followed by a DATA command, followed by arbitrary RCPT commands and a second DATA command.
https://exchange.xforce.ibmcloud.com/vulnerabilities/42658
http://www.vupen.com/english/advisories/2008/1684/references