OnTLSNotAvailable is a published TIdOnTLSNegotiationFailure property in TIdNNTP that represents the event handler triggered when using TLS (Transport Layer Security) and TLS support is not available. 

OnTLSNotAvailable is triggered after the connection is established in Connect, and when UseTLS indicates optional TLS suppport is implied. 

The event handler accepts a Boolean argument that indicates if the client connection can continue after execution of the event handler.