Just to add a little more info to this old post, I would never insert a comma between Hi and someone's name. My emails always look more or less like this:
Hi Mary,
How are you? Bleblebleh.
Take care,
Ingrid
I only know one person who inserts a comma between the word "hi" and the name, and that person's native language is not English. I was going to correct him, but Googled this topic first to make sure I wasn't wrong. Turns out I am wrong! Grammatically, it is correct to use the comma, according to the grammar notes I found on other websites.
Nevertheless, even though it is grammatically correct to use the comma, I do not plan on using it. It does not seem to be used by most people and ends up looking incorrect as a result.