If you need to take a certain hardware node offline for maintenance or upgrade you can bring a Hardware Node into a special maintenance mode that temporarily disables management of Containers or Plesk domains hosted on this Hardware node from the Control Panel.
To bring a node into the maintenance mode, go to the Service Director - Plesk Manager - Nodes or Virtuozzo Manager - Nodes. Select a Hardware Node and edit its general settings.
Note that you can compose a Mass Mailer for customers on that specific Hardware Node.