How to disable mail service.
Mail service may be disabled right in Plesk Panel GUI. Go
'Tools & Settings -> Services Management', find there your mail server title and turn it off. But note that mail service will be started again after reboot.
To disable mail functions completely, it is necessary:
Uninstall mail server (QMail, Postfix) from your OS. The simple way is to use Plesk Panel GUI. Go
'Tools & Settings -> Updates & Upgrades -> Add or Remove components', find there your mail Server and uninstall it
In Windows Plesk Panel there is an opportunity to select no mail server available. Follow
'Tools & Settings -> Mail Server Settings -> Install and use another mail server'and select