Applies to HMS v3.0 and above
You can disable clear text login by adding the below config value to the MTA'a main config file.
The default config directory is located here : C:\Program Files\Hurricane MTA Server\config
In that directory you would open the general.config file in a text editor and add the below config section. You can place it anywhere but the bottom works best. Once you make the change you will need to restart the MTA services. This will disable clear text unless the connection is secure.
Login = TlsOnly