Saying 'Hi all' isn't improper English, but it's not commonly heard either. I've heard it when people are being silly (very informal) and are among friends. I usually only address 'all' in a forum post if I'm also writing specific lines to specific people, like this: Hello all, here's something that pertains to everybody!
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Is "Hi Bilbo" or "Hi, Bilbo" both correct when starting an email, or is one correct/preferred? For example:
Hi, Bilbo.
Hi Bilbo