Next month Microsoft will be changing the default behaviour for LDAP – Cleartext, unsigned LDAP queries against AD (over port 389) will be disabled by default - https://support.microsoft.com/en-gb/help/4520412/2020-ldap-channel-binding-and-ldap-signing-requirement-for-windows  .  You'll still be able to over-ride that using registry keys or group policy, but the best advice is to configure all LDAP clients to use encrypted, signed LDAPS queries (over port 636).