If a "File Filtering Rule" is created to block attachments by extension of file name, it will not block messages where the attachment is zero (0) bytes in size. This issue is observed on Exchange 2007 and 2010. Exchange 2003 is not affected.
Content Filtering within Symantec Mail Security for Microsoft Exchange (SMSMSE) will only work under one of the following conditions:
VSAPI message body scanning is enabled.
There is content in the message, which means a message body or attachment.
VSAPI is only invoked when there is something to scan. When no message body or attachment exists, or if the attachment is 0 bytes, then the message is passed through and ignored by VSAPI. To properly test or trigger rules that have been implemented, insert text into the message body or add an attachment with a size count over 0 bytes.
Symantec is currently investigating this problem and will update this knowledge base article when new information becomes available. Please subscribe to receive an email notification upon changes made to this article.
Create an Exchange Server (2007/2010) Transport Rule which silently drop all emails with zero-bytes attachments.