source: http://www.securityfocus.com/bid/6923/info

Microsoft Outlook and Outlook Express may execute arbitrary programs through objects embedded in HTML email messages. When an email message or newsgroup message is viewed using Outlook, a temporary object is created in the Internet Explorer cache. The security zone of this cache should be set by Internet Explorer and is set to the Internet Zone by default.

It is possible to have an object embedded in an HTML message reference an executable file using a CODEBASE reference and non-zero CLASSID value. It may also be possible for an attacker to place a file in a known temporary folder and have it executed through this method.

An issue similar to this was reported for Internet Explorer (BID 3867). It appears that the issue was fixed for objects viewed in a web page through Internet Explorer, but not through HTML email viewed with Outlook or Outlook Express. 

<xml id=oExec> <security><exploit> <![CDATA[ <object id="oFile"
classid="clsid:11111111-1111-1111-1111"
code base="C:WINDOWSFTP.EXE"></object>]]></exploit></security></xml>
<SPAN dataFld=exploit dataFormatAs=html
dataSrc=#oExec></SPAN>