How to handle possibly subtly flirtatious emails from students? I got a series of emails from a student. At first, I thought she was being over-polite. But now things are making me very uncomfortable. For instance, at every email she thanks me for giving her good grades; it gave her motivation, encouragement etc. (this makes me very uncomfortable since it might imply I'm giving her preferential treatment) and I am the best TA she has ever seen. Her emails bear a tone which can be interpreted as either overly polite or very subtly flirtatious e.g. using emoticons, signing emails with only "Yours", "have a good night", "lovely day".

I am willing to give the student the benefit of doubt. English may not be her first language, so she may not know some phrases in emails are only appropriate with your closed ones. I want to tell her that she should avoid these phrases in formal emails not so directly. How can I convey this to the student?
You should keep your replies short, polite and professional. Do not respond to what you see as flirtatious. Try also to interpret what you see as flirtatious in a different light, as you say, it could be due to language issues. It would be bad if you started to respond in a manner that assumed something that was not intended.

In the end it is very important to not fuel any behaviour that you think is "suspect" but at the same time you cannot avoid responding altogether. Acknowledge praise briefly but do not return praise since that could appear as favouring a student before others. Turn the focus of a response quickly from any polite exchange to focus on the course material. You may also take opportunities to point out that information given is also given to others to at least subtly impress the fact that there is no "special treatment".

In short, act as if nothing special is going on, be brief to the point and professional in your response. Do not try to be anything other than yourself or treat any student differently from others. Keep records of your mail or other exchanges.