I am not a native speaker. I really want to know what's the difference between Hi and Hey in writing an email? In daily conversation? Is it too girlish or childish for a man to say hey in his email? What's the bottom line in using them?
Hey or Heya are much less formal. You could use them if you were writing to friends.
Hey Dave, not spoken to you in a while. How're things? What've you been up to?
You could possibly use it in a more casual email (like a single question) to a colleague that you speak to often:
Hey Bob, just wondering if you've had a reply from Customer X yet?
Hi is the standard greeting for office e-mails. If you're not sure, stick with Hi.
Thanks for the information on [foo]. Do you know whether item 10 is for all customers or just Y?
I'll get back to you on [question] when I've had more time to look at the data.
You could also use Hi for friends.
Hey or Heya can be used to greet people you already know. Friends or colleagues. It's warmer than Hi so probably isn't suited to colleagues that you're less close to.
Hey Dave, glad you could make it. How're you?
Hey guys, what's up?
Hi can be used to greet people you already know, or those you don't.
Hi Bob, how's Sylvia?
Hi Kate, nice to meet you. How do you know Dan?
There is no gender differentiation in who can say either term, or who they are said to.
Hi is used to greet someone and is a shortened version of hello. "Hello George". "Hi Mary". Hey was used to beckon someone. "Hey boy! Come here!". It is in recent times that Hi became too formal whilst meeting friends and Hey somehow found it's way into such colloquial meetings. "Hey Mary, How have you been?" If you are going to be using the written word or an email, stick to using Hi.
Speaking as a native english speaker living in england, I occasionally greet my friends with any of: Hi, Hey, Hola, Yo, Dobradien, Ciao, or Wazzup, although of course never bonjour (I am english, after all)*.
In other words, when casually greeting a friend, or even when meeting someone in a casual setting, you can say pretty much anything. In an email or other written context, I would always use "Hi" or something more formal ("Dear X", "To whom it may concern", etc).
Nowadays, "Hello" tends to sound a little stilted, although no one would fault you for using it, especially if you have an accent.
Supplementary to this, I present for your consideration: hot dog squirrel comet
* This is a joke. England and France have a long history/tradition of rivalry.
My observations are that hey is rapidly overcoming hi as an informal greeting, originally on a personal level, but increasingly in business environments where the parties already know each other. There is a certain robustness to greeting a friend or colleague with "hey," making "hi" sound a bit effete--it is interesting that the original questioner got that backwards, thinking that hey sounded girlish. BTW, the British commenters are not really qualified to comment on a question of evolving usage like this, since they speak British, which is a dialect, rather than English. Current Standard English means American English. The language of Shakespeare, referring to current, articulate, vigorous, expressive use of the English language, means American English, not what is spoken on the increasingly insignificant little island where the language happened to originate.