In reply to someone else's email, you decide you wish to either quote something, or perhaps rebut what has been said. Like real mail, email has an envelope. This envelope is, however, unlike any other mail envelope you have ever encountered. If you examine the screen shots shown in figures 5.3 and 5.4 you will note that this is where the primary difference lies. When sending email vs. posting to a Newsgroup, the envelopes are different.
Lets deal with the envelope in parts so that each section becomes clear.
We know that in order to send email to someone, we need to know their address. If you want the email to be received by more than one person you can also include additional email addresses here. Just separate them by commas. It's major use is to send a duplicate copy of the mail to one or more secondary individuals. For example lets say you wanted to send an email to your boss, reminding him about your vacation.