When a user opens an Outlook email or previews the email in one of the Outlook panels, the OLE mechanism will automatically read the embedded Flash object and try to execute it, to provide a preview.

Since most Flash exploits only need to be executed to work, and because there’s a flaw in the Outlook security sandboxing system, an attacker can easily embed malicious Flash objects inside emails and have other malicious code executed via older (Flash) vulnerabilities.

Source: BadWinmail Microsoft Outlook Bug Can Give Attackers Control Over PCs