• your audience is at a distance: The fact that email is an online medium means that distance is no object. It will be much cheaper than a long distance call and faster than the snail mail.
• your want time to compose your message; Use email in preference to a 'phone call or in-person meeting, if you compose your message carefully'.
• You require a record your message: whether it’s stored on disk or printed out, an email cen give you a permanent record of your message, something that’s more difficult to achieve with the ‘phone or working face-to face’