Unscannable File Rule

I'm using SMSMSE version 6.5.7.279 on Exchange Server 2007. Every now and then when a user forwards an email with attachments, they get quarantined. I get an email notification with the following message in the body:

Location of the message:  SMTP
Sender of the message: JDoe@OurDomain.com Subject of the message:  Fwd: Pricing

The attachment(s) "Pricing Sheet.docx" and/or the message was Quarantined.

This was done due to the following Symantec Mail Security settings:
Scan: Auto-Protect
Rule: Unscannable File Rule

Why do I keep getting unscannable file rule and how do I fix this problem?