InterScan Messaging Security Suite (IMSS) Windows is unable to connect to the LDAP server via ports 3269 and 636.
This is a product limitation.
LDAP SSL uses ports 3269 and 636 but IMSS Windows does not support LDAP SSL. The table shows the ports used by LDAP and LDAP SSL services/protocols:
|Global Catalog LDAP||None||3268|
|Global Catalog LDAP SSL||None||3269|
For more information about Active Directory and how it works, refer to the Microsoft article How Active Directory Searches Work.