To forward yor email to another mail server other than the hosting mail server
you need to follow the followinig steps:-
1) Log in to Plesk control panel.
2) Click the "Domains" shortcut in the navigation panel.
3) Click the required domain name in the list.
4) Click on "DNS settings" (Under website category).
5) Click on host containing the MX record of your domain name.
6) In the mail exchanger box you need to enter the name of your mail server. Make sure to
click on "OK" once you have made the required changes.
NOTE:- The above changes may come into effect after some time so we
recommend you to wait for atleast 24 hours and then check.