How to handle 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 (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 tone which can be interpreted as either overly polite or very subtly flirtatious.

I am willing to give the student benefit of doubt. English mayn't be her first language, so she mayn't 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?