Answer
There was a bug where extremely badly formatted and very non-RFC compliant multi-part MIME messages missing the closing boundary for inner multipart/alternative section were able to get through.
This has been fixed in builds 3136 and above. Additionally if you have GMS Anti-Spam (3136 or above) there is an option to block badly formatted messages such as this using the new "message quality" tab.
See Also:
Keywords:anti virus bypass error