To enable Atmail from the plesk panel :-
1. Login 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 Mail Accounts > Mail Settings.
5) Select the dropdown menu for Webmail and select the client "Atmail" for webmail and hit the "OK" button at the bottom right on the panel.