This is a somewhat subtle option. If the option is checked, then a command to delete the message(s) will be added to the queue. The delete will be executed when any current activity is completed. If this option is unchecked, then the message(s) won't be deleted until the next time the messages' accounts are checked. This is more efficient than deleting immediately.

Note: This option is available on the "Configuration" page only when "Enable Advanced Options" is checked (also on the "Configuration" page)