In Symantec Messaging Gateway (SMG) allows to configure Directory Data Sources (such as LDAP) to perform certain operations such as recipients validation, address resolution, authentication and routing.
SMG also offers a setting called "Enable Distribution Expansion" which allows Distribution Lists to be 'expanded' when performing address validation.
This function does not support Directory Data Sources configured for routing.
Messaging Gateway (SMG)
The appliance is currently working as per its design and the Distribution Expansion only supports Address Resolution.