Clearswift MIMEsweeper 5.0.5, when it has been upgraded from MAILsweeper for SMTP version 4.3 or MAILsweeper Business Suite I or II, allows remote attackers to bypass scanning by including encrypted data in a mail message, which causes the message to be marked as "Clean" instead of "Encrypted".
https://exchange.xforce.ibmcloud.com/vulnerabilities/18035
http://www.securityfocus.com/bid/11669
http://secunia.com/advisories/13160
http://download.mimesweeper.com/www/TechnicalDocumentation/MSWSMTP505UpdateReadMe.htm