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Professors will typically have the university email account completely swamped with hundreds of mails from students, conferences, announcements of papers, other interesting stuff, people who press reply-to-all when normal reply would be sufficient, invitations for workshops, newsletters, FYI-emails, and also some important stuff. You'll end up on the not-so-important list, if you don't end up in the spam. That's not an insult to you, that's just the scientific community spamming itself to death.
All good points. Especially PIs of large collaborative projects can be swamped by hundreds of mails per day (I have seen that personally). Adding to that admininistrative mails and student's mails from existing courses, there is simply not enough time to answer even most of them.
Also there are people who basically mass-mail people with requests to join labs, become graders etc (I got a sizeable amount of those myself). Thus, if the mail even looks remotely like a mass-mail/spam (e.g. by being generic and impersonal) it will end up in trash mostly unread.
If you really want to join someone, research him/her, ask for a time for a call, or, if possible, show up during office hours. In other words, make it personal. Also, try to leave a positive impression, but do not try too hard (no one likes the guy who pretends to be a know-it-all).