I only use a greeting if it's a formal sort of e-mail.
If it's someone I know, they'll recognize my address, or else their e-mail program will indicate who it is. "Hi" - I never really care for greetings anyway. The presence of the e-mail there should be greeting enough. Greetings are more of a formality to me. Though I guess they serve as a sort of, "Alert, I'm about to start talking, so start listening." If you just start talking to someone, it's like a TCP/IP packet without a header. The first packet will get dropped because the receiver wasn't ready. Then it's just something like, "Hey," but that's only if I'm talking, not e-mailing. The recipient opened the e-mail, so there's no need for the "header" part of the conversational TCP/IP packet, if you get my metaphor.
I love using metaphors like that.
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