From Lunarpages Web Hosting Wiki
An e-mail alias is a way of mapping a name to an actual e-mail address. This sort of thing is very easy to setup on your hosting plan with Plesk.
How to Setup an E-mail Alias in Plesk
To setup an e-mail alias, the first thing you need to do is login to your Plesk control panel and then click on the domain name for the e-mail alias you wish to create. Next, click on the icon labeled "Mail".
This will take you to the mail names for your domain page. Locate and click on the e-mail account for which you would like to create an alias. Next, click on "Add New Mail Alias".
Into the empty text box, enter the name of the new alias, then hit the "OK" button. Your new e-mail alias should now be setup and ready to forward mail from one e-mail address to another one without the need of create two separate e-mail accounts.