UsingTLS is a read-only Boolean property that indicates if the connection for the client context is using TLS (Transport Layer Security). 

UsingTLS returns True when the IOHandler for the client context is derived from TIdSSLIOHandlerSocketBase. Encrypted secure IOHandlers are created for POP3 server connections when TIdPOP3Server.UseTLS contains a value that indicates TLS is optional (using the POP3 STARTTLS command) or required. 

UsingTLS in used in the command handler that implements support for the POP3 STARTTLS command.

TIdSSLIOHandlerSocketBase TIdPOP3Server.UseTLS