Symptoms

Domain example.com is assigned to IP address 203.0.113.2

When trying to open a domain that does not exist in Plesk like mail.example.com or a server hostname, browser shows the content of website example.com

Cause

Website example.com is set as the default site for the IP address 203.0.113.2. When trying to open mail.example.com or hostname, the request gets redirected to an IP address that domain resolves to, thus, the content of IP's default site is shown.

Resolution

Set None in Tools & Settings > IP Addresses > 203.0.113.2 > Default site

In this case Plesk default page will be shown when trying to open a domain that does not exist in Plesk, or a server hostname (if it resolves to the server IP globally).