Sending a short message on the computer with Fetion always shows that sending a message failed.

Well, if there is nothing wrong with your network, there are several situations. One is that if the other party doesn't open a flying letter, you can't send it. If the other party stops, you can't send it. If the other party has a fetion, you can receive it even if you turn it off without stopping, which is equivalent to a short message sent by a computer.