You are receiving a large number of unwanted emails that are not being detected as Spam or Suspected Spam. Many of these emails appear to be Newsletters or Marketing type emails.
Symantec Mail Security for Microsoft Exchange (SMSMSE) by default only monitors for messages classified strictly as Spam messages. Other unwanted messages types such as Bulk marketing emails, Newsletter emails, and suspicious URLs are not identified.
These features are available in SMSMSE 7.5.4 and later. When these features are enabled, the messages meeting these criteria are classified as "Suspected Spam" and the action for Suspected Spam under Policies -> Premium AntiSpam Actions is taken.
To enable detection of Marketing Emails, News letter Emails, or Suspicious URLs: