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