SMTP Routing recipient verification against AD/LDAP - no Server available

Sophos UTM: SG430 (9.506-2)

I want to enable in the SMTP Routing tab the recipient verification against the Active Directory.

But the big problem is, if I choose Active Directory I can't choose a server from the given dropdown list.

Even though I created several entries:

• SSL Enabled/Disabled
• Various AD Service Accounts
• With/Without Base DN given
• HA Group  
• Single host

Final result: A dropdown with the option "None"

How do you manage the recipient verification, do you use the AntiSpam Agent on Exchange via the Edge Transport role?
Main problem is, our appliance is mostly on 100% CPU load because of the SMTPD service works on bounced emails.

Thanks in advance!