Zero byte .tmp files are left in the SMSMSE temp (default location C:\Program Files (x86)\Symantec\SMSMSE\7.0\Server) folder when the "Quarantine Entire Message" action is taken against a message.
This issue is resolved in SMSMSE 7.5.0 and 7.0.3. Upgrade to 7.5.0 or 7.0.3 to resolve this problem.
Configure all enabled policies to use the "Quarantine attachment/message body and replace with text" action. This will prevent buildup of 0 byte files.
Alternatively, periodically clear out the 0 byte files from the directory until a more permanent solution is available.