LDAP acceptance validation is a feature that allows the Cisco Secure Email Gateway to check if the recipient address of an incoming message exists in an LDAP directory before accepting it. This feature can help prevent spammers from sending emails with invalid recipient addresses and reduce the load on the appliance2. Reference = User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Configuring LDAP Queries [Cisco Secure Email Gateway] - Cisco