Well, I haven't received an email from the complaining student yet.
Sometimes I don't understand those students. One student emailed me, saying, "I have concerns about my essay and want to talk in person," and I replied, "OK, then come over during my office hours," and the student never showed up. Where did his concerns go? One student sent her paper via email, "I'm intimated by the professor, so please let me hand this in to you." Oh, man, you should know that the electric version is not acceptable. That kind of stuff.
I gradually understand that university teachers are having a hard time to handle those young students.