1. Disable my network interface.
2. telnet 202.108.9.33 25, in "dos" window.
* the above ip address is a public mail service provider locally.

The phenonmenon make me confused: the windows turns all black, and with a cursor flashing on the left upper corner of the window, then back to the prompt line. without a word.
In common senses, when the network is break, it will show error message when trying to do this.

But if in this following case, it's different, and it would be recognised as a normal response.
1. Disable my network interface.
2. Open Kaspersky window--setting--service--network settings--port settings---uncheck the "general smtp".ok--apply---ok.
3. telnet 202.108.9.33 25, in "dos" window.
Here is an error information.

Anyone who can explain this to me is highly appreciated!!