On project task created do not send email
I found that if project has notifications option "
Visible by following customers" enabled then one can configure subscription for each follower. 
To not receive e-mails when new task is added to the project: unmark the checkbox "Task opened" in the "Edit subscription of User" form.
By default subscribers get email messages once the new task in a project is created. How it can be tailored so that unless the projects has checkbox "Send e-mail on new task" checked it will not send e-mails on new task?
I know how to add a custom field to project.project model. But don't know the next step.
What action to override to not send the email when a new task is created and "Send e-mail on new task" is not checked for project?