Here is what i have set under Profile -> LDAP (5.4.2 fw)
Name : RR-LAB-LDAP
ServerIP: 188.8.131.52 port 389 non-secure
Bind DN: CN=fortimail service,OU=Service Account,DC=RR-LAB,DC=co,DC=uk
Bind Password: ********
Then when you press Browse, should show your AD structure.
Under "User Query Options" ensure you choose the correct schema for your directory service, eg MS Active Directory
Under "User Authentication Options" I have "Search user and try bind DN" selected
Under "Advanced Options" Ive set LDAPv3 and password schema Active Directory
These settings work for both recipient verification and webmail authentication.