The "Manage Domains" → "Domain Aliases" page allows you to create a list of aliases for a certain domain.
A domain alias is a secondary domain directed to the domain you are editing. For example, if you are currently editing "domain1" and you add "domain2" as an alias, all emails sent to "account@domain2" will be delivered to "account@domain1".
Each of the previously defined domain aliases can be edited using the text field listed under "Address". To delete an alias, use its corresponding "Delete" button. To add a new alias, type its name in the upper right corner text field and click the "Add Alias" button.
When you are done configuring these parameters, remember to click the "Save Configuration" button to preserve your changes.