I always greet those who reply my questions with "Dear Tutor". I also found that there were other greetings, like "Dear Teacher". Are these both right? And there is another word "mentor", which seems have the same meaning. Could you also tell me whether it could be used in the beginning of a letter?
Most importantly, how would you like us to greet you?
I wouldn't use them in a letter- use a name or Sir/Madam. I wouldn't say them in posts either- using the name makes more sense to me. However, many cultures find such directness difficult, so use what you feel comfortable with. I would prefer it if people called me Tdol, but I really don't mind at all if they want to use teacher/tutor/sensei, etc.