Multiple unspecified vulnerabilities in the SMTP service in MailEnable Standard Edition 1.x, Professional Edition 3.x and earlier, and Enterprise Edition 3.x and earlier allow remote attackers to cause a denial of service (crash) via crafted (1) EXPN or (2) VRFY commands.
https://www.exploit-db.com/exploits/5235
https://exchange.xforce.ibmcloud.com/vulnerabilities/41083
https://euvd.enisa.europa.eu/vulnerability/EUVD-2008-1283
http://www.vupen.com/english/advisories/2008/0800/references
http://www.securityfocus.com/bid/28154