Why is it that when someone sends an email ,it bounces back

In computer jargon, a bounced <a href="http://www.webopedia.com/TERM/E/e_mail.html">e-mail</a> is one that never arrives in the recipient's inbox and is sent back, or bounced back, to the sender with an error message that indicates to the sender that the e-mail was never successfully transmitted. But what happens when someone sends an e-mail out into cyberspace, and why do e-mails sometimes bounce back?<br /> When a <a href="http://www.webopedia.com/TERM/U/user.html">user</a> attempts to send an e-mail, he is telling his e-mail system to look for the <a href="http://www.webopedia.com/TERM/D/domain.html">domain</a> of the recipient (for example, webopedia.com) and the domain's mail <a href="http://www.webopedia.com/TERM/S/server.html">server</a>. Once the e-mail system makes contact with the recipient's mail server, the mail server looks at the message to determine if it will let the message pass through the server. If the recipient's server has predetermined that it is not accepting e-mails from the sender's address (for example, if it has blocked the address for <a href="http://www.webopedia.com/TERM/S/spam.html">anti-spamming</a> purposes), the server will reject the message and it will subsequently bounce back to the sender. The message will also bounce back to the server if the mail server on the recipient's end is busy and cannot handle the request at that time. When an e-mail is returned to the sender without being accepted by the recipient's mail server, this is called a <a href="http://www.webopedia.com/TERM/H/hard_bounce.html">hard bounce</a>.