Example:

Your company, XYZ, Inc, has 5 employees, Linda, Mary, Harry, John, and Bob. You have 5 mailboxes assigned to your account (xyz01, xyz02, xyz03, xyz04 and xyz05). Your domain name is xyz.com. You assigned xyz01 to Linda, xyz02 to Mary, xyz03 to Harry, xyz04 to John and xyz05 to Bob. Linda is the sales manager so you assigned her the E-mail address sales@xyz.com by adding the alias sales@xyz.com to xyz01. Linda also wants a personal E-mail address so you add alias linda@xyz.com to xyz01. Mary is a customer service specialist so you assigned service@xyz.com to xyz02 mailbox. You assigned support@xyz.com to Harry's mailbox (xyz03) and so forth... You are now ready for receiving E-mail. If someone sends an email to sales@xyz.com, it will arrive in Linda's mailbox (xyz01). Likewise, if someone sends an E-mail to linda@xyz.com, it will also arrive in Linda's mailbox (xyz01). When Linda checks her mail, she should receive 2 messages, one addressed to sales@xyz.com and one addressed to linda@xyz.com.

Six months later Linda is no longer working for the company so Mary has to take over her position until your company hires a new employee to replace Linda, you need to redirect all incoming mail to sales@xyz.com to Mary's mailbox. To do this, you need to delete the alias sales@xyz.com and then add sales@xyz.com to Mary's mailbox (xyz02). Now if someone sends an email to sales@xyz.com, it will arrive in Mary's mailbox. Since Linda is no longer with the company, you may also delete the alias linda@xyz.com.

Sue is the new employee that was hired to replace Linda. You need to give her a mailbox. Since Linda left the company, mailbox xyz01 is no longer in use. You assigned this mailbox to Sue (don't forget to change the password so Linda will not be able to read Sue's E-mail). You need to go through the process of add/remove alias to assign an E-mail address to Sue.