Prevent ActionMailer from sending to deleted users / blacklisted addresses

No need to check in normal code, just do everything as usual and this interceptor will prevent you from spamming people who do not want any emails.
Its not perfect(missing bc/bcc filters but if should be fine for 90% of cases)

#config/initializers/blacklisted_emails.rb
class MailInterceptor
  def self.delivering_email(message)
    if User.where(:email => message.to, :receive_emails => false).any?
      message.perform_deliveries = false
    end
  end
end

Mail.register_interceptor(MailInterceptor)