It is a pain when emails are sent to nonexistent users. There are two options you can choose for this kind of situations, either reject all the emails or forward it to another address. To set up a catchall email account in Plesk follow the below steps.
Login to your Plesk panel.
Select Mail tab.
Click on Change Settings icon.
Specify the required details in that form.
Select Forward to address if you want the emails to be forwarded.
Specify the email address and click OK to complete.