UseTLS is a published Boolean property used to indicate if Transport-Level Security is used on the connection to the SMTP server. When UseTLS is True, transport-level encryption and/or authentication is used for the connection. 

Changing the value of UseTLS to True forces the value in UseEHLO to to be set to True, as well. UseEHLO is a required when explicit TLS is requested and the server supports the capability. 

Use Capabilities to determine if the server supports use of StartTLS prior to performing authentication for the SMTP session.