How to write at the beginning of English consultation letter?

Usually, dear sir/madam.

In fact, when I contacted the school before, as long as you knew the name of the person in the department you want to write, you could just write: Hello, XXX, this (your name), I want to know.

My tutor or something once wrote me an email, that's all.

If you want to be formal and formal, who cares?