You have enabled the blank subject and sender content filtering rule in Symantec Mail Security for Exchange (SMSMSE), but messages with no subject, sender, or message body are still making it through the filter and being delivered to users.
SMSMSE has trouble identifying these blank subject and sender messages using the \s* regular expression if the message body is blank. As soon as content is added to the message body, the rule will begin to trigger, but because these message bodies are completely blank, the rule does not function as expected.
Change the regular expression in the main Rule tab from \s* to ^$ and save the changes. Save the rule, and deploy the changes. The rule will now take action as configured against completely blank messages.