SMTP service in SmarterTools SmarterMail 1.6.1511 and 1.6.1529 allows remote attackers to cause a denial of service (CPU consumption) via a large number of simultaneous open connections to TCP port 25.
https://exchange.xforce.ibmcloud.com/vulnerabilities/15391
http://www.zone-h.org/advisories/read/id=4098
http://secunia.com/advisories/11042
http://members.lycos.co.uk/r34ct/main/smarter_mail%203.1/smarter_mail.txt