The e-mail application is flawed in the way it handles Windows Address Book files. Opening a specially crafted WAB file can result in execution of malicious code, giving an attacker control of the Windows PC.
The Bat! is very well-protected against viruses and worms that spread via email. The Bat! doesn't use the Windows address book , which has been the target of many recent attacks.
You can find more information about this flaw here.