Mailbox alias properties are located in CONTROL PANEL > Users. Click the user's display name, then click the user's email address.

Aliases are multiple references to a single mailbox. They allow you to receive (but not send) email from alternate email addresses. Aliases must be in a domain associated with your Exchange account, for example sales@yourdomain.com and info@yourdomain.com. Mail sent to the alias email address will be delivered to the mailbox of the Primary Address for the user.

All mail sent out by the user will have the Primary Address in the From field of the message.

Note: Aliases as well as mailbox names must be unique for each YOUR COMPANY account.