This article talks about how you can configure your server group policy settings to allow authentication to work with a Lightweight Directory Access Protocol (LDAP) server.
You cannot connect to Active Directory (AD) through the Smoothwall. You have enabled LDAP server security signing requirements on the local AD.
This issue is caused by Samba not supporting the LDAP server signing requirements that have been configured on your AD via a group policy. As a result, the AD terminates the connection the Smoothwall is trying to make, preventing the Smoothwall Filter & Firewall from looking up group information for users.
There is a Microsoft knowledge base article which explains in more detail why this group policy may be incompatible with some applications: http://support.microsoft.com/kb/823659.
Configure the server group policy settings to None. You can find this security setting by opening the appropriate policy and expanding the console tree: Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options. This will need to be done at the top of the domain for the global policy.
Data signing is not required to bind with the server, and at present Smoothwall does not support it.