Using 'hello' in a greeting

Recently, when trying to write with a very formal style but not feeling comfortable saying 'dear,' I have been addressing some emails or letters like this -

Hello, Mr. Smith:

Or if I feel less formal, I write something like this -

Hi, Mr. Smith:

I was told that this was the correct way to use the word 'hello' or 'hi' while retaining formality. Of course the semicolon could be replaced with an exclamation point if I wanted to. I was also told that this is not a common usage and few people know about it, but it is actually the correct way to write.

Is this true? I would appreciate any feedback.

Thank you,
D.