Prevent a Mailer Delivery in Rails

example.md

class NotificationsMailer < ApplicationMailer
  # https://guides.rubyonrails.org/action_mailer_basics.html#action-mailer-callbacks  
  after_action :prevent_delivery

  def unread
    @user = params[:user]
    mail(to: @user.email, subject: 'Here's what you missed...')
  end

  private
    
    def prevent_delivery
      mail.perform_deliveries = @user.should_perform_delivery?
    end
end

class User < ApplicationRecord
  ...
  def should_perform_delivery?
    # @user.should_perform_delivery?
    # => false
  end
end