MAILsweeper for SMTP 4.3.6 and 4.3.7 allows remote attackers to cause a denial of service (CPU consumption) via a PowerPoint attachment that either (1) is corrupt or (2) contains "embedded objects."
https://exchange.xforce.ibmcloud.com/vulnerabilities/12052
http://www.securityfocus.com/bid/7562
http://www.clearswift.com/download/bin/Patches/ReadMe_SMTP_438.htm