Suppose you have 5 employees, and you set each up with a mail box.
Now we set up each user with email addresses. Each box can have more than one address going to it.
You can also have email sent to more than one account.
Sometimes you will also forward an address to an external E-mail
You can also have several domains pointing to the same email box.
You can also have ALL mail at your domain pointing to one email box.
This is often referred to a catchall or slush account.
For help on Email setup