A business application called CaseWare from CaseWare International sends files as attachments via e-mail. When scanned by Symantec Mail Security for Microsoft Exchange (SMSMSE) the attachment receives an "unscannable" verdict. The attachments are quarantined or deleted depending on the action configured.
SMSMSE attempts to open the attachment and scan the individual parts. SMSMSE is not handling the CaseWare file formats and marking the attachment as unscannable.
This issue is resolved in SMSMSE 6.5.5, upgrade to SMSMSE 6.5.5 to resolve this problem.
See the following article for potential workarounds: What criteria can cause an "unscannable file" violation within Symantec products for Microsoft Exchange?