Hi,
Yes
Help says:
Click to drop messages addressed to user names that do not exist in your LDAP directory. No bounce messages are returned to the sender. You must configure a data source for recipient validation before enabling this feature.
or SMG_Admin_Guide, Page 109
If you choose to drop invalid recipients, Symantec Messaging Gateway drops
from the mail stream any messages that are addressed to email addresses that
do not exist in your LDAP directory. No bounce messages are returned to the
sender. You must have a data source configured for recipient validation.