Clearswift MAILsweeper for SMTP before 4.3.20 and MAILsweeper for Exchange before 4.3.20 allows remote attackers to bypass the "text analysis", possibly bypassing SPAM and other filters, by sending an e-mail specifying a non-existent or unrecognized character set.
https://exchange.xforce.ibmcloud.com/vulnerabilities/27301
http://www.vupen.com/english/advisories/2006/2473
http://www.securityfocus.com/bid/18584
http://secunia.com/advisories/20756
http://download.mimesweeper.com/www/TechnicalDocumentation/ReadMe_MSW_4%2C3%2C20.htm