To add email forwarding in Plesk,please follow the following steps :-
1) Login to Plesk control panel.
2) Click on "Subscription".
3) Click on "Manage Hosting" in front of the domain whose files permession you want to change.
3) Click on the "Mail Tab" and "Select email address" for whome you want to set the "forwarding".
5) Click on "Forwarding Tab".
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.
