For plesk version 10 and above
=========================
1. Log into plesk control panel.
2. Select the particular domain from "Domains" list.
3. Select the "Mail" tab from the window.
4. Select the email address.
5. Select "Forwarding" tab.
6. Enable the check box for "Switch on mail forwarding" option.
7. Enter the destination email addresses in the box.
8. Click on "OK" button.
For plesk version 9
===============
1. Log into plesk control panel.
2. Select the particular domain from "Domains" list.
3. Select the option "Mail Accounts" from "Mail" tab.
4. Select the email address.
5. Select the "Redirect" option from "Tools" tab.
6. Enable the check box for "Redirect" option.
7. Enter the destination email address in "Redirect address" field.
8. Click on "OK" button.