How to disable Redmine email notifications for the whole system
Set config.action_mailer.perform_deliveries in config/application.rb to false. Redmine then sends no email notifications at all. The settings in "Administration" → "Settings" → "Email notifications" and in "My account" are ignored.
config.action_mailer.perform_deliveries = false
Note that this is a general setting for Ruby on Rails applications, and not only for Redmine.
