Exchange Sender ID filter issues after installing Symantec Messaging Gateway

Recently installed Symantec Messaging Gateway at 2 different locations - one Exchange 2010, one Exchange 2013.  I left all Exchange filtering enabled assuming it was good to have a 2nd layer of spam filtering.

When investigating some false positives at both locations I'm finding that most are senders that are failing Exchange's built-in Sender ID check filter.  These senders were not failing this check before, and no changes were made on the Exchange side of things.  Both locations are running the latest Symantec Messaging Gateway.

Anyone have any idea on this?  Does having SMG installed as a gateway filter somehow cause senders to be more likely to fail Exchange's Sender ID check?  Should Sender ID filtering be disabled on Exchange?

Thanks!