Presence, or the idea of knowing whether your friend is there to receive your message, ranks as the main difference between e-mail and IM [source: The National Archives]. When you send e-mail, the message heads to the recipient's inbox, and you don't know when it will be read (unless you request a receipt that notifies you when the recipient has opened your e-mail). IM is more immediate. If you can see that your friend is online, you know you might get a quick response.
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