The section Mail (along with the option Mail Server Settings) is not shown in Plesk in Tools & Settings.
The button Mail is missing in Domains > example.com.
Postfix mail server is in use.
The following command executed in the server shell shows the below output:
# plesk bin mail -i
Mail service is not installed in Plesk.
One of the mail components is not properly installed and/or not correctly detected by Plesk.
Note: The solution is intended for use only by server administrators. In case there is no administrative access to Plesk, contact the server administrator or server provider.
Go to Tools & Settings > Server Components and click Refresh:
If the issue still persists, perform the following actions: