To add email forwarding in Plesk,please follow the following steps :-
1) Login to Plesk control panel.
2) Click on "Domains" and then Click on "Control panel" in front of the domain whose files permession you want to change.
3) Click on the "Mails".
4) Click on email address for whome you want to set the forwarding.
5) Click on "Forwarding".
6) Click on check box in front of "Switch on mail forwarding"and enter the
email addresses on which you want to set the email forwarding.
7) Click on "OK" once you have entered the email addresses on which you want to set the forwarding.