vpopmail 5.4.14 and 5.4.15, with cleartext passwords enabled, allows remote attackers to authenticate to an account that does not have a cleartext password set by using a blank password to (1) SMTP AUTH or (2) APOP.
https://exchange.xforce.ibmcloud.com/vulnerabilities/26333
http://www.vupen.com/english/advisories/2006/1698
http://www.securityfocus.com/bid/17894
http://sourceforge.net/project/shownotes.php?release_id=415350