If you are not available to answer emails, for example due to a vacation, you can switch on an automatic response system in plesk that will respond to the people who have sent you emails with a message of your choosing.
You can switch it on by following these steps:
1. Choose the server where your email is.
2. Log in
3. Click on "Change email account settings" and after that, the tab "Auto-Reply" (Other way to do it: Email addresses -> Click on your email address -> Auto-Reply)
4. Click OK when you are pleased with the message. The auto-reply system will work after that.