Hello Paul,
I'm sorry but SMTP AUTH with LDAP is not supported by the CSVB. Only SMTP AUTH via Forward (GUI: Network -> SMTP Authentication -> Add Profile) is possible. In this case, the presented AUTH string will be forwarded in a new SMTP session to a (backend) mail server for authentication.
To enable SMTP Authentication, you'll need first of all a SMTP Authentication profile, which needs to be configured on the respective Listener configuration (GUI: Network -> Listeners -> (your listener name) -> SMTP Authentication Profile). Once this step has been completed, you'll need to turn on SMTP Authentication in the respective Mail Flow Policies (GUI: Mail Policies -> Mail Flow Policies -> (Policy in question) -> SMTP Authentication: preferred or required depending on your needs).
Hope this helps.
Regards,
Martin