Apologies for the lack of response here. If you haven't already found out, the way to do this is configure the ldap authenticator in Catalogs > Authenticators (and not System > Users > Authenticators). Then you can use the auth.query() function to query it with the user/password (and see the auth.query() docs for an example with HTTP), then probably connection.close("...") with the right SMTP syntax to reject connections.
This article might be helpful in extracting the user/password from SMTP traffic: https://community.pulsesecure.net/t5/Pulse-Secure-vADC/HowTo-Inspect-and-synchronize-SMTP/ta-p/29185.
If you need further assitance writing the Trafficscript then Pulse Secure can help but this would be under a professional services engagment, please see https://kb.pulsesecure.net/articles/Pulse_Secure_Article/KB44677.
Pulse Secure vADC Support (Acquired by Ivanti)